docs: more fixes for afdocs

[jessiemongeon1]

⏺ Summary of all changes in ~/move-book:

Fix 2 — content-negotiation:

• site/src/plugins/llms-txt.ts: Added content-negotiation middleware that
  checks Accept: text/markdown and serves .md files
• site/serve-with-rewrites.js: New production test server with content
  negotiation

Fix 3 — markdown-code-fence-validity:

• reference/index-syntax.md:104: Fixed 4-backtick typo to 3 backticks
• site/src/plugins/llms-txt.ts: Added stripHtmlComments() to remove HTML
  comments from exported markdown (fixed the struct-methods.md
  fence-inside-comment issue)

Fix 4 — llms-txt-coverage:

• book/sidebar.yml: Added link to Guides category, uncommented
  open-sourcing-libraries, added 6 programmability pages
• reference/sidebar.yml: Made Abilities a category with abilities/object
  sub-item
• site/src/plugins/llms-txt.ts: Added /search page to llms.txt and generates
  search.md

Fix 5 — llms-txt-directive:

• site/src/plugins/llms-txt.ts: Moved
  move.ebnf/move-semantics.md/llms-full.txt into the blockquote description so
  they aren't sampled as doc pages for directive checking

[damirka]

llms-txt is alright, I'm not sure how the "serve-with-rewrites.js" is used

[jessiemongeon1]

@damirka serve-with-rewrites.js is used for content negotiation where the same URL returns different content based on Accept: text/markdown.