feat(web): advertise the Markdown .md variant for discoverability

Add <link rel="alternate" type="text/markdown" href=".../<key>.md"> to every
doc page head (gated by markdownDocs; workers inherit the env), and document
the .md convention in llms.txt. Agents can now discover the Markdown variant
without sniffing — it serves at /docs/<key>.md under its own cache key.