Add API docs for instance level variables

doc/api/admin_ci_instance_level_variables.md

+# Instance-level CI/CD variables API
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14108) in GitLab 13.0
+
+## List all instance variables
+
+Get the list of all instance-level variables.
+
+```plaintext
+GET /admin/ci/variables
+```
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables"
+```
+
+```json
+[
+    {
+        "key": "TEST_VARIABLE_1",
+        "variable_type": "env_var",
+        "value": "TEST_1",
+        "protected": false,
+        "masked": false
+    },
+    {
+        "key": "TEST_VARIABLE_2",
+        "variable_type": "env_var",
+        "value": "TEST_2",
+        "protected": false,
+        "masked": false
+    }
+]
+```
+
+## Show instance variable details
+
+Get the details of a specific instance-level variable.
+
+```plaintext
+GET /admin/ci/variables/:key
+```
+
+| Attribute | Type    | required | Description           |
+|-----------|---------|----------|-----------------------|
+| `key`     | string  | yes      | The `key` of a variable |
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables/TEST_VARIABLE_1"
+```
+
+```json
+{
+    "key": "TEST_VARIABLE_1",
+    "variable_type": "env_var",
+    "value": "TEST_1",
+    "protected": false,
+    "masked": false
+}
+```
+
+## Create instance variable
+
+Create a new instance-level variable.
+
+```plaintext
+POST /admin/ci/variables
+```
+
+| Attribute       | Type    | required | Description           |
+|-----------------|---------|----------|-----------------------|
+| `key`           | string  | yes      | The `key` of a variable. Max 255 characters, only `A-Z`, `a-z`, `0-9`, and `_` are allowed. |
+| `value`         | string  | yes      | The `value` of a variable. |
+| `variable_type` | string  | no       | The type of a variable. Available types are: `env_var` (default) and `file`. |
+| `protected`     | boolean | no       | Whether the variable is protected. |
+| `masked`        | boolean | no       | Whether the variable is masked. |
+
+```shell
+curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables" --form "key=NEW_VARIABLE" --form "value=new value"
+```
+
+```json
+{
+    "key": "NEW_VARIABLE",
+    "value": "new value",
+    "variable_type": "env_var",
+    "protected": false,
+    "masked": false
+}
+```
+
+## Update instance variable
+
+Update an instance-level variable.
+
+```plaintext
+PUT /admin/ci/variables/:key
+```
+
+| Attribute       | Type    | required | Description             |
+|-----------------|---------|----------|-------------------------|
+| `key`           | string  | yes      | The `key` of a variable.   |
+| `value`         | string  | yes      | The `value` of a variable. |
+| `variable_type` | string  | no       | The type of a variable. Available types are: `env_var` (default) and `file`. |
+| `protected`     | boolean | no       | Whether the variable is protected. |
+| `masked`        | boolean | no       | Whether the variable is masked. |
+
+```shell
+curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables/NEW_VARIABLE" --form "value=updated value"
+```
+
+```json
+{
+    "key": "NEW_VARIABLE",
+    "value": "updated value",
+    "variable_type": "env_var",
+    "protected": true,
+    "masked": true
+}
+```
+
+## Remove instance variable
+
+Remove an instance-level variable.
+
+```plaintext
+DELETE /admin/ci/variables/:key
+```
+
+| Attribute | Type    | required | Description             |
+|-----------|---------|----------|-------------------------|
+| `key`     | string  | yes      | The `key` of a variable |
+
+```shell
+curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables/VARIABLE_1"
+```

doc/api/api_resources.md

@@ -112,6 +112,7 @@ The following API resources are available outside of project and group contexts
 
 | Resource                                           | Available endpoints                                                     |
 |:---------------------------------------------------|:------------------------------------------------------------------------|
+| [Instance-level CI/CD variables](admin_ci_instance_level_variables.md) | `/admin/ci/variables`                               |
 | [Admin Sidekiq queues](admin_sidekiq_queues.md)    | `/admin/sidekiq/queues/:queue_name`                                     |
 | [Appearance](appearance.md) **(CORE ONLY)**        | `/application/appearance`                                               |
 | [Applications](applications.md)                    | `/applications`                                                         |