Support llms.txt and plain Markdown for AI coding agents

With the rise of AI-assisted coding, it would be highly beneficial to provide documentation in formats optimized for LLMs. I propose adding support for an `/llms.txt` file and providing a way to access documentation as plain Markdown.

Start by tracing how this site publishes documentation and how a root-level /llms.txt entry point could be generated or served. Then identify the existing documentation entry points and build output needed for plain-Markdown access; done means both /llms.txt and documentation in plain Markdown are available as described.

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