Add API documentation

doc/api/api_resources.md

@@ -126,6 +126,7 @@ The following API resources are available outside of project and group contexts
 | [Runners](runners.md)                             | `/runners` (also available for projects)                                |
 | [Search](search.md)                               | `/search` (also available for groups and projects)                      |
 | [Settings](settings.md)                           | `/application/settings`                                                 |
+| [Statistics](statistics.md)                       | `/application/statistics`                                               |
 | [Sidekiq metrics](sidekiq_metrics.md)             | `/sidekiq`                                                              |
 | [Suggestions](suggestions.md)                     | `/suggestions`                                                          |
 | [System hooks](system_hooks.md)                   | `/hooks`                                                                |

doc/api/statistics.md

+---
+table_display_block: true
+---
+
+# Application statistics API
+
+## Get current application statistics
+
+List the current statistics of the GitLab instance.
+
+```
+GET /application/statistics
+```
+
+```bash
+curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/application/statistics
+```
+
+Example response:
+
+```json
+{
+   "forks": "10",
+   "issues": "76",
+   "merge_requests": "27",
+   "notes": "954",
+   "snippets": "50",
+   "ssh_keys": "10",
+   "milestones": "40",
+   "active_users": "50"
+}
+```
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