Docs: Add example to upload a project avatar via API

doc/api/projects.md

@@ -1927,6 +1927,38 @@ The returned `url` is relative to the project path. The returned `full_path` is
 the absolute path to the file. In Markdown contexts, the link is expanded when
 the format in `markdown` is used.
 
+## Upload a project avatar
+
+Uploads an avatar to the specified project.
+
+```plaintext
+PUT /projects/:id
+```
+
+| Attribute | Type           | Required               | Description |
+|-----------|----------------|------------------------|-------------|
+| `avatar`  | string         | **{check-circle}** Yes | The file to be uploaded. |
+| `id`      | integer/string | **{check-circle}** Yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding). |
+
+To upload an avatar from your file system, use the `--form` argument. This causes
+cURL to post data using the header `Content-Type: multipart/form-data`. The
+`file=` parameter must point to an image file on your file system and be 
+preceded by `@`. For example:
+
+Example request:
+
+```shell
+curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" --form "avatar=@dk.png" "https://gitlab.example.com/api/v4/projects/5"
+```
+
+Returned object:
+
+```json
+{
+  "avatar_url": "https://gitlab.example.com/uploads/-/system/project/avatar/2/dk.png"
+}
+```
+
 ## Share project with group
 
 Allow to share project with group.