Add agent-friendly support plan and phase 1 implementation of Markdown output

[chalin]

• Contributes to:
  • Add support for agent-friendly content generation #2596
  • Add golden tests #726
• Adds task/plan docs for the new feature and its golden-test coverage strategy.
• Adds experimental opt-in Markdown output support to Docsy via layouts/all.md and a new Markdown output format.
• Enables Markdown outputs on docsy.dev for home, page, and section; keeps search.md HTML-only.
• Uses a simple fallback rendering model: title, description blockquote, rendered body content, and section child links.
• Preserves existing HTML, RSS, and print outputs.
  • Adds golden-based tests for Markdown output
  • Adds docsy.dev Markdown golden tests plus a manifest-driven update script.
  • Wires Markdown golden verification into docsy.dev's npm test.
• Defers llms.txt, content negotiation, taxonomy/term rollout, and Markdown-specific link rewriting to later phases.

Preview:

• Home: HTML | Markdown
• Docs home: HTML | Markdown
• Get started: HTML | Markdown
• Content: HTML | Markdown
• Blog index: HTML | Markdown
• Blog post: HTML | Markdown
• French home: HTML | Markdown

Code diffs are only due to new alternate links, and build/page timestamps:

$ (cd public && git diff -bw --ignore-blank-lines -I 'link rel="alternate' -- ':(exclude)*.xml') | grep ^diff       
diff --git a/fr/project/index.html b/fr/project/index.html
diff --git a/fr/search/index.html b/fr/search/index.html
diff --git a/project/index.html b/project/index.html
diff --git a/search/index.html b/search/index.html
$ (cd public && git diff -bw --ignore-blank-lines -I 'link rel="alternate|buildDate|dateModified|modified_time' -- ':(exclude)*.xml')
$ # no output

$ curl -sI https://deploy-preview-2597--docsydocs.netlify.app/index.md
HTTP/2 200 
accept-ranges: bytes
age: 0
cache-control: public,max-age=0,must-revalidate
cache-status: "Netlify Edge"; fwd=miss
content-type: text/markdown; charset=UTF-8