Add spec document for .well-known/agent-skills/ discovery

Distilled from Cloudflare's Agent Skills Discovery RFC. Covers the
`index.json` format, `$schema` versioning, URL resolution, SHA-256
digest verification, archive distribution (`.tar.gz`/`.zip`), HTTP
requirements, and client implementation steps.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/docs.json

@@ -19,6 +19,7 @@
       "home",
       "what-are-skills",
       "specification",
+      "well-known-uri",
       {
         "group": "For skill creators",
         "pages": [