docs: rewrite doc-spec, slim README, add migration guides

[swetjen]

Replace the aspirational doc-spec with an enforceable contract, slim the README from 707 to ~195 lines, and add router-migration documentation.

• doc-spec.md: rewrite ALL-CAPS plaintext into spec-compliant Markdown that obeys its own rules. Drop unbuilt machinery (SSG, JSON agent-index, versioned URL trees) into an explicit "deferred" section; keep a lighter required frontmatter schema, heading discipline, gofmt-clean language-tagged code, and GitHub-native callouts.
• README.md: collapse the two competing "Why RPC" sections into one, add a constraints-as-features table, fix the broken main() indentation, resolve the httpapi voice to "HTTP-native library" (not "legacy"), and move both cookbooks into the docs tree.
• docs/rpc/patterns.md, docs/http-legacy/patterns.md: new cookbook pages holding the RPC and httpapi advanced patterns extracted from the README.
• docs/tutorials/coming-from-routers.md: new guide for porting gin/echo/chi/ fiber/net-http services, with a concept map, before/after recipes, an accessor map, and an agent port prompt.
• overview.md, tutorials/overview.md: point at the new migration guide and fill the tutorials index.