Docs: add index for project operations

doc/user/project/operations/index.md

+# Project operations
+
+GitLab provides a variety of tools to help operate and maintain
+your applications:
+
+- Collect [Prometheus metrics](../integrations/prometheus_library/index.md).
+- Deploy to different [environments](../../../ci/environments.md).
+- Connect your project to a [Kubernetes cluster](../clusters/index.md).
+- Discover and view errors generated by your applications with [Error Tracking](error_tracking.md).
+- Create, toggle, and remove [Feature Flags](https://docs.gitlab.com/ee/user/project/operations/feature_flags.html). **[PREMIUM]**
+- [Trace](https://docs.gitlab.com/ee/user/project/operations/tracing.html) the performance and health of a deployed application. **[ULTIMATE]**