Improve MR Analytics documentation
What does this MR do?
The Manage:Analytics group accidentally created 2 documentation MRs for the Merge Request Analytics feature:
- !39522 (merged) was created earlier, but missed important details
- !39604 (closed) was created later, but contained those important details.
Due to time constraints we merged 1 and closed 2, so this MR takes those important details from 2 and adds them to the documentation.