Add documentation as tool for chat

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/414642

Add documentation as tool for chat

This uses TanukiBot to anwer questions about GitLab when being asked in the chat.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

1. Set up the local environment for Anthropic and OpenAI https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally
2. Set up embeddings https://docs.gitlab.com/ee/development/ai_features.html#set-up-the-embedding-database
3. Enable the feature flag Feature.enable(:gitlab_duo), Feature.enable(:tanuki_bot)
4. Go to Help -> Ask GitLab Chat

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.