Add the sphinx-llms-txt docs plugin for better LLM integration

[Dev-iL]

Add the sphinx-llms-txt extension to Hamilton's Sphinx documentation build. This auto-generates llms.txt (a compact index) and llms-full.txt (complete docs in one Markdown file) llms-txt on every documentation deploy.

The plugin supports code file inclusion, custom summaries, and size limits. Readthedocs Since Hamilton already uses Sphinx + MyST on Read the Docs, integration requires only adding the extension to conf.py and configuring sections. This gives every LLM provider access to current, version-specific Hamilton documentation through a standardized URL.

Changes

• Add sphinx-llms-txt to pyproject.toml and docs/conf.py
• Remove outdated sphinx dependency pins

How I tested this

Built the docs locally.

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.