docs: add sphinx-llms-txt extension

[claw-explorer]

Summary

Add the sphinx-llms-txt extension to the documentation build. This generates a summary llms.txt file and a single combined documentation llms-full.txt file for AI/LLM consumption of the documentation.

Changes

• Added sphinx-llms-txt>=0.7.1 to the docs dependency group in pyproject.toml
• Added sphinx_llms_txt to the Sphinx extensions list in docs/conf.py

Verification

Built the documentation locally and confirmed that both llms.txt and llms-full.txt are generated successfully:

sphinx-llms-txt: Created llms-full.txt with 109 sources and 6825 lines
sphinx-llms-txt: created llms.txt

The build completes with no new warnings.

Closes #1302

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📚 Documentation preview 📚: https://icalendar--1303.org.readthedocs.build/

[read-the-docs-community]

Documentation build overview

📚 icalendar | 🛠️ Build #32020110 | 📁 Comparing 9bc08a3 against latest (8d9046d)

🔍 Preview build

Show files changed (2 files in total): 📝 2 modified | ➕ 0 added | ➖ 0 deleted

File	Status
404.html	📝 modified
reference/changelog.html	📝 modified

[niccokunzmann]

https://icalendar--1303.org.readthedocs.build/en/1303/llms.txt

It works.