Add AI features development navigation item

What does this MR do and why?

I noticed that we don't yet have a navigation item for development/ai_features.md

CC @tmccaslin

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/ee/development/ai_features.html
• After: https://eread-add-ai-development-navigation-item.docs.gitlab-review.app/ee/development/ai_features.html

How to set up and validate locally

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

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request 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 merge request.