Add AI tooling guide and link it from user index

[ericholscher]

Motivation

• Provide a practical guide to make documentation more discoverable, citeable, and maintainable for AI assistants by documenting best practices and signals that improve automated retrieval and citation.

Description

• Add a new documentation page docs/user/ai-tooling.rst that explains AI-ready documentation practices such as publishing llms.txt, preserving version context and stable URLs, exposing sitemaps, controlling agent access, improving content extraction quality, and automating AI-related workflows.
• Update docs/user/index.rst to include the new /ai-tooling page in the Hosting documentation toctree so the guide is discoverable in the site navigation.
• The new guide links to existing reference pages (/reference/llms-txt, /reference/sitemaps, /reference/robots, etc.) and lists common pitfalls to avoid.

Testing

• Performed a local Sphinx build (make html) to verify the new page and index changes compile into the documentation successfully.

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Codex Task