AI Experiment documentation

What does this MR do and why?

This merge request will lay the foundation for all the AI Experiment feature documentation. We want this unified under a single page for now, we eventually will transition this into the various areas of documentation as these features mature.

Review app: https://main-ee-118149.docs.gitlab-review.app/ee/user/ai_features.html

Describe in detail what your merge request does and why.

Screenshots or screen recordings

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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.