Add HTML-to-markdown generation for llms.txt

[svekars]

Add llm_generate_md theme option that converts Sphinx HTML output to clean .md files and links to them in llms.txt instead of .html files.
Also generates llms-full.txt with all page content concatenated.

• Add _html_to_markdown() with BeautifulSoup (+ regex fallback)
• Strip theme-injected metadata (date info, headerlinks, nav elements)
• Normalize Unicode punctuation to ASCII for compatibility
• Generate llms-full.txt per the llms.txt spec
• Register llm_generate_md option in theme.conf - True by default
• Add 33 tests covering conversion, file generation, and edge cases
• Add .claude/ to .gitignore

Example generated .md files:
HTML: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/community/build_ci_governance
.md: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/community/build_ci_governance.md

HTML: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/community/design
.md: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/community/design.md

llms.txt: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/llms.txt
llms-full.txt: https://deploy-preview-239--pytorchsphinxtheme.netlify.app/llms-full.txt

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