Add Composer API documentation

doc/api/packages/composer.md

+---
+stage: Package
+group: Package
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Composer API
+
+This is the API documentation for [Composer Packages](../../user/packages/composer_repository/index.md).
+
+WARNING:
+This API is used by the [Composer package manager client](https://getcomposer.org/)
+and is generally not meant for manual consumption.
+
+For instructions on how to upload and install Composer packages from the GitLab
+package registry, see the [Composer package registry documentation](../../user/packages/composer_repository/index.md).
+
+NOTE:
+These endpoints do not adhere to the standard API authentication methods.
+See the [Composer package registry documentation](../../user/packages/composer_repository/index.md)
+for details on which headers and token types are supported.
+
+## Base repository request
+
+Returns the repository URL templates for requesting individual packages:
+
+```plaintext
+GET group/:id/-/packages/composer/packages
+```
+
+| Attribute | Type   | Required | Description |
+| --------- | ------ | -------- | ----------- |
+| `id`      | string | yes      | The ID or full path of the group. |
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/group/1/-/packages/composer/packages"
+```
+
+Example response:
+
+```json
+{
+  "packages": [],
+  "metadata-url": "/api/v4/group/1/-/packages/composer/p2/%package%.json",
+  "provider-includes": {
+    "p/%hash%.json": {
+      "sha256": "082df4a5035f8725a12i4a3d2da5e6aaa966d06843d0a5c6d499313810427bd6"
+    }
+  },
+  "providers-url": "/api/v4/group/1/-/packages/composer/%package%$%hash%.json"
+}
+```
+
+This endpoint is used by Composer V1 and V2. To see the V2-specific response, include the Composer
+`User-Agent` header. Using Composer V2 is recommended over V1.
+
+```shell
+curl --user <username>:<personal_access_token> \
+     --header "User-Agent: Composer/2" \
+     "https://gitlab.example.com/api/v4/group/1/-/packages/composer/packages"
+```
+
+Example response:
+
+```json
+{
+  "packages": [],
+  "metadata-url": "/api/v4/group/1/-/packages/composer/p2/%package%.json"
+}
+```
+
+## V1 packages list
+
+Given the V1 provider sha, returns a list of packages within the repository. Using Composer V2 is
+recommended over V1.
+
+```plaintext
+GET group/:id/-/packages/composer/p/:sha
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | string | yes | The ID or full path of the group. |
+| `sha`     | string | yes | The provider sha, provided by the Composer [base request](#base-repository-request). |
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/group/1/-/packages/composer/p/082df4a5035f8725a12i4a3d2da5e6aaa966d06843d0a5c6d499313810427bd6"
+```
+
+Example response:
+
+```json
+{
+  "providers": {
+    "my-org/my-composer-package": {
+      "sha256": "5c873497cdaa82eda35af5de24b789be92dfb6510baf117c42f03899c166b6e7"
+    }
+  }
+}
+```
+
+## V1 Package Metadata
+
+Returns the list of versions and metadata for a given package. Using Composer V2 is recommended over
+V1.
+
+```plaintext
+GET group/:id/-/packages/composer/:package_name$:sha
+```
+
+Note the `$` symbol in the URL. When making requests, you may need to use the URL-encoded version of
+the symbol `%24` (see example below).
+
+| Attribute      | Type   | Required | Description |
+| -------------- | ------ | -------- | ----------- |
+| `id`           | string | yes      | The ID or full path of the group. |
+| `package_name` | string | yes      | The name of the package. |
+| `sha`          | string | yes      | The sha digest of the package, provided by the [V1 packages list](#v1-packages-list). |
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/group/1/-/packages/composer/my-org/my-composer-package%245c873497cdaa82eda35af5de24b789be92dfb6510baf117c42f03899c166b6e7"
+```
+
+Example response:
+
+```json
+{
+  "packages": {
+    "my-org/my-composer-package": {
+      "1.0.0": {
+        "name": "my-org/my-composer-package",
+        "type": "library",
+        "license": "GPL-3.0-only",
+        "version": "1.0.0",
+        "dist": {
+          "type": "zip",
+          "url": "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=673594f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "reference": "673594f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "shasum": ""
+        },
+        "source": {
+          "type": "git",
+          "url": "https://gitlab.example.com/my-org/my-composer-package.git",
+          "reference": "673594f85a55fe3c0eb45df7bd2fa9d95a1601ab"
+        },
+        "uid": 1234567
+      },
+      "2.0.0": {
+        "name": "my-org/my-composer-package",
+        "type": "library",
+        "license": "GPL-3.0-only",
+        "version": "2.0.0",
+        "dist": {
+          "type": "zip",
+          "url": "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=445394f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "reference": "445394f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "shasum": ""
+        },
+        "source": {
+          "type": "git",
+          "url": "https://gitlab.example.com/my-org/my-composer-package.git",
+          "reference": "445394f85a55fe3c0eb45df7bd2fa9d95a1601ab"
+        },
+        "uid": 1234567
+      }
+    }
+  }
+}
+```
+
+## V2 Package Metadata
+
+Returns the list of versions and metadata for a given package:
+
+```plaintext
+GET group/:id/-/packages/composer/p2/:package_name
+```
+
+| Attribute      | Type   | Required | Description |
+| -------------- | ------ | -------- | ----------- |
+| `id`           | string | yes      | The ID or full path of the group. |
+| `package_name` | string | yes      | The name of the package. |
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/group/1/-/packages/composer/p2/my-org/my-composer-package"
+```
+
+Example response:
+
+```json
+{
+  "packages": {
+    "my-org/my-composer-package": {
+      "1.0.0": {
+        "name": "my-org/my-composer-package",
+        "type": "library",
+        "license": "GPL-3.0-only",
+        "version": "1.0.0",
+        "dist": {
+          "type": "zip",
+          "url": "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=673594f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "reference": "673594f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "shasum": ""
+        },
+        "source": {
+          "type": "git",
+          "url": "https://gitlab.example.com/my-org/my-composer-package.git",
+          "reference": "673594f85a55fe3c0eb45df7bd2fa9d95a1601ab"
+        },
+        "uid": 1234567
+      },
+      "2.0.0": {
+        "name": "my-org/my-composer-package",
+        "type": "library",
+        "license": "GPL-3.0-only",
+        "version": "2.0.0",
+        "dist": {
+          "type": "zip",
+          "url": "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=445394f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "reference": "445394f85a55fe3c0eb45df7bd2fa9d95a1601ab",
+          "shasum": ""
+        },
+        "source": {
+          "type": "git",
+          "url": "https://gitlab.example.com/my-org/my-composer-package.git",
+          "reference": "445394f85a55fe3c0eb45df7bd2fa9d95a1601ab"
+        },
+        "uid": 1234567
+      }
+    }
+  }
+}
+```
+
+## Create a package
+
+Create a Composer package from a Git tag or branch:
+
+```plaintext
+POST projects/:id/packages/composer
+```
+
+| Attribute | Type   | Required | Description |
+| --------- | ------ | -------- | ----------- |
+| `id`      | string | yes      | The ID or full path of the group. |
+| `tag`     | string | no       | The name of the tag to target for the package. |
+| `branch`  | string | no       | The name of the branch to target for the package. |
+
+```shell
+curl --request POST --user <username>:<personal_access_token> --data tag=v1.0.0 "https://gitlab.example.com/api/v4/projects/1/packages/composer"
+```
+
+Example response:
+
+```json
+{
+  "message": "201 Created"
+}
+```
+
+## Download a package archive
+
+Download a Composer package. This URL is provided in the [v1](#v1-package-metadata)
+or [v2 package metadata](#v2-package-metadata)
+response. A `.zip` file extension must be in the request.
+
+```plaintext
+GET projects/:id/packages/composer/archives/:package_name
+```
+
+| Attribute      | Type   | Required | Description |
+| -------------- | ------ | -------- | ----------- |
+| `id`           | string | yes      | The ID or full path of the group. |
+| `package_name` | string | yes      | The name of the package. |
+| `sha`          | string | yes      | The target sha of the requested package version. |
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=673594f85a55fe3c0eb45df7bd2fa9d95a1601ab"
+```
+
+Write the output to file:
+
+```shell
+curl --user <username>:<personal_access_token> "https://gitlab.example.com/api/v4/projects/1/packages/composer/archives/my-org/my-composer-package.zip?sha=673594f85a55fe3c0eb45df7bd2fa9d95a1601ab" >> package.tar.gz
+```
+
+This writes the downloaded file to `package.tar.gz` in the current directory.

doc/user/packages/composer_repository/index.md

@@ -13,6 +13,9 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 Publish [Composer](https://getcomposer.org/) packages in your project's Package Registry.
 Then, install the packages whenever you need to use them as a dependency.
 
+For documentation of the specific API endpoints that the Composer
+client uses, see the [Composer API documentation](../../../api/packages/composer.md).
+
 ## Create a Composer package
 
 If you do not have a Composer package, create one and check it in to