docs: add llm.txt file

[mscolnick]

        # This is technically off by 1 commit, but generating the llm.txt file is easiest from a
        # deployed site.
        # We also ignore txt and xml files
        run: npx sitefetch https://docs.marimo.io -o site/llm.txt --match "!**/*.{txt,xml}"

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[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.10.14-dev17