Add Code Review Analytics documentation

What does this MR do?

This adds a documentation page for the Code Review Analytics feature, and a link to it on the Analytics index page. Related to Issue #38062 (closed).

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