Fern moves docs to static prebuilt pages

In short

Fern Docs sites are now prebuilt as static pages and served from the edge; custom domains still pointing to Vercel must move to cname.ferndocs.com by 2026-12-01 or publishing will fail. Fern generates and maintains llms.txt automatically and links the OpenAPI spec in it for sites with API endpoints, and the AsyncAPI spec for WebSocket channels.