Add docs for Code Review AI Experiments

What does this MR do and why?

Describe in detail what your merge request does and why.

This adds documentation for the Code Review AI experiments that are currently enabled:

• Summarize my merge request review
• Summarize merge request changes
• Generate tests in merge requests

Related: #409730 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.