Merge branch 'docs-kubernetes-api-40473-ee' into 'master'

EE port for API project cluster documentation

See merge request gitlab-org/gitlab-ee!9057

doc/api/README.md

@@ -56,6 +56,7 @@ The following API resources are available:
   - [Projects](projects.md) including setting Webhooks
   - [Project access requests](access_requests.md)
   - [Project badges](project_badges.md)
+  - [Project clusters](project_clusters.md)
   - [Project-level variables](project_level_variables.md)
   - [Project import/export](project_import_export.md)
   - [Project members](members.md)