Document Review Apps

What does this MR do?

This adds documentation for Review apps as they are implemented now (based on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6665).

What are the relevant issue numbers?

Closes #51201 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• Conforms to the code review guidelines
• Conforms to the style guides