[Infra] Add text/markdown output; enable AI agent discovery

[vitorvasc]

• I have read and followed the Contributing docs, especially the "First-time contributing?" section.
• This PR has content that I did not fully write myself.
  • I used AI and I have read and followed the Generative AI Contribution Policy.
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

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Closes #9449

• Add a text/markdown output format so every page gets an index.md alternative with rendered content and no layout chrome (nav, sidebar, footer, scripts)
• Add llms.txt at the site root as a discovery index for AI agents, listing docs and recent blog posts with links to their markdown versions

Inspired by this guide.

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR, without using AI. ↩