add nginx_status monitoring details

doc/administration/monitoring/index.md

@@ -7,3 +7,4 @@ Explore our features to monitor your GitLab instance:
 - [GitHub imports](github_imports.md): Monitor the health and progress of GitLab's GitHub importer with various Prometheus metrics.
 - [Monitoring uptime](../../user/admin_area/monitoring/health_check.md): Check the server status using the health check endpoint.
   - [IP whitelists](ip_whitelist.md): Configure GitLab for monitoring endpoints that provide health check information when probed.
+- [nginx_status](https://docs.gitlab.com/omnibus/settings/nginx.html#enabling-disabling-nginx_status): Monitor your Nginx server status