Improve MR Analytics documentation

What does this MR do?

The Manage:Analytics group accidentally created 2 documentation MRs for the Merge Request Analytics feature:

1. !39522 (merged) was created earlier, but missed important details
2. !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.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content