feat(docs): add llms.txt and Markdown page versions

[henryiii]

🤖 AI text below 🤖

Port of scikit-hep/uhi#252. Adds LLM-friendly docs output with NVIDIA's sphinx-llm (sphinx_llm.txt extension). It runs the markdown builder alongside the HTML build and merges the results, so the whole thing is one extension and one build:

• llms.txt — sitemap with a one-line description per page.
• llms-full.txt — all docs in one file (5 MB here, given the API pages).
• <page>.html.md next to each <page>.html.

llms_txt_description is set because the default summary is the full README. index.rst gets a meta description, which llms.txt uses for the index page.

[ariostas]

Looks good to me. Thank you, @henryiii! (Please merge if you're not planing to make any more changes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.40%. Comparing base (1c06db0) to head (884f1c2).
⚠️ Report is 54 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

see 47 files with indirect coverage changes