Make docs discoverable by LLMs and coding agents (sphinx-llm)

[aaronspring]

What & why

As coding agents become a primary way developers discover and use libraries, they read our documentation and write code based on what they find. This PR makes xskillscore's docs easy for LLMs/agents to discover and parse, following the recommendations from Jacob Tomlinson's PyData talk "Marketing your open source project to LLMs" (https://youtu.be/bRDtplyhRYw).

Changes

• Generate llms.txt, llms-full.txt and per-page .md during the Sphinx HTML build via NVIDIA's sphinx-llm extension (sphinx_llm.txt). It runs the markdown builder under the hood to emit true markdown following the llms.txt standard:
  • llms.txt — a sitemap with per-page descriptions
  • llms-full.txt — the entire docs concatenated as one context-efficient markdown file
  • index.html.md, etc. — a markdown version of every page at the same URL + .md
  • llms_txt_description sets the project summary; the title comes from the project name.
• Documentation breadcrumbs in xskillscore/__init__ — a module docstring pointing at the docs, the llms.txt/llms-full.txt files, and the source repo, so an agent reading the installed source is guided to the right places.
• Build deps: add sphinx-llm to ci/doc.yml and the complete optional-dependencies extra (it pulls in sphinx-markdown-builder).

After this is merged and built on Read the Docs, the files will be available at:

• https://xskillscore.readthedocs.io/en/stable/llms.txt
• https://xskillscore.readthedocs.io/en/stable/llms-full.txt

Verification

• Confirmed sphinx-llm (0.4.1) generates llms.txt, llms-full.txt and per-page .md with the exact config used here (sphinx_llm.txt + llms_txt_description) in an isolated Sphinx build.
• import xskillscore works with the new module docstring.
• ruff check / ruff format --check pass; conf.py compiles.

Note: a full local make html currently fails on an unrelated, pre-existing incompatibility between the newest Sphinx and sphinx-autosummary-accessors; the conda-pinned RTD/CI environment is unaffected.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (e457985) to head (9ae20d2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
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- Coverage   94.57%   94.50%   -0.08%     
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  Files          27       27              
  Lines        2841     2841              
==========================================
- Hits         2687     2685       -2     
- Misses        154      156       +2     

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[Zeitsperre]

LGTM