Merge branch 'docs/add-merge-trains-to-api-res' into 'master'

Link merge_trains.md from docs/api/api_resources.md

See merge request gitlab-org/gitlab!41453

doc/api/api_resources.md

@@ -44,6 +44,7 @@ The following API resources are available in the project context:
 | [Members](members.md)                                               | `/projects/:id/members` (also available for groups)                                                                                                                                                   |
 | [Merge request approvals](merge_request_approvals.md) **(STARTER)** | `/projects/:id/approvals`, `/projects/:id/merge_requests/.../approvals`                                                                                                                               |
 | [Merge requests](merge_requests.md)                                 | `/projects/:id/merge_requests` (also available for groups and standalone)                                                                                                                             |
+| [Merge trains](merge_trains.md)                                     | `/projects/:id/merge_trains`                                                                                                                                                                          |
 | [Notes](notes.md) (comments)                                        | `/projects/:id/issues/.../notes`, `/projects/:id/snippets/.../notes`, `/projects/:id/merge_requests/.../notes` (also available for groups)                                                            |
 | [Notification settings](notification_settings.md)                   | `/projects/:id/notification_settings` (also available for groups and standalone)                                                                                                                      |
 | [Packages](packages.md)                                             | `/projects/:id/packages`                                                                                                                                                                              |