YEF-1062: advertise docs AI discovery headers

[Yevanchen]

Summary

• Add shared docs discovery headers for Content-Signal, root llms.txt, docs llms.txt, and full docs llms-full.txt.
• Apply those headers to docs markdown route handlers and to static-exported markdown assets through the Cloudflare worker.
• Add regression coverage for docs HTML and markdown discovery responses.

Validation

• env TMPDIR=/tmp npm run test:seo in mosoo-docs
• env TMPDIR=/tmp npm run build in mosoo-docs
• npm run test:seo in mosoo-website

Closes YEF-1062