[tracker] Improve support for AI-agent doc consumption

[chalin]

This is a meta issue to track work per phase:

• Phase 1: core structural support via llms.txt and Markdown output; aim AFDocs
  score of >= 80% for participating sites.
  • Add support for agent-friendly content generation, phase 1 #2596.
• Phase 2:
  • Improve llms.txt and Markdown output; increase AFDocs score, with associated qualitative metrics. Aim AFDocs score of >= 90%
  • Initial work in creating md versions of shortcodes
  • Exploration of creating md versions of traditionally HTML-heavy pages like the homepage.
  • Document __hugo_ctx issues and proposals to deal with it (e.g., OTel.io does post processing to keep dev builds as efficient as possible).
  • Potential topics to address:
    • Heading {# ...} Markdown-syntax extension
    • ...
• Phase 3: TBD

At some point we're looking into some form of semantic assessment. Possibly more on that later.

Related (at least partly)

• AI chat for documentation #2421