feat(docs): make docs more agent friendly

[tharropoulos]

Change Summary

• preserve frontmatter in generated markdown instead of stripping it
• add description frontmatter to overview pages
• add description frontmatter to guide pages
• add description frontmatter to latest API reference pages for 30.1
  and 30.2
• emit per-language .md variants for pages with tabbed code samples for the latest two versions
• generate llms.txt with grouped documentation links, page descriptions, latest version info, language-variant guidance, and the OpenAPI spec link
• generate llms-full.txt by concatenating cleaned markdown content from docs pages
• warn when llms-full.txt exceeds 2 MB
• prepend a visible llms.txt hint to served markdown responses
• inject a hidden llms.txt hint into rendered docs pages after MarkdownActions

PR Checklist

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