docs: document the Metadata type, typed page props, JSON-LD, and preconnect on the docs site

[vivek7405]

Problem

The typed-metadata and typed-route surfaces are complete in agent-docs/ but missing from the two docs-site pages an author actually lands on:

• docs/app/docs/typescript/page.ts never mentions the Metadata type (Export a Metadata type for metadata / generateMetadata returns #257), nor PageProps / LayoutProps / RouteHandlerContext, webjs types, or .webjs/routes.d.ts (Generate typed route props and a route union for navigate()/<a href> #258).
• docs/app/docs/metadata-routes/page.ts shows a bare untyped export const metadata = {...} (no Metadata annotation), has no JSON-LD jsonLd coverage (Add JSON-LD structured-data support to the metadata API #260, which currently appears only on the SSR page), and no metadata.preconnect / metadata.dnsPrefetch fields (Content-hash asset URLs for immutable cache, add preconnect hints #243, the preconnect half of asset hints).

Implementation notes (for the implementing agent)

• Where to edit:
  • docs/app/docs/typescript/page.ts: add a "Typed metadata and page props" section covering import type { Metadata } from '@webjsdev/core', PageProps<R> / LayoutProps<R> / RouteHandlerContext<R>, the webjs types command, and the generated .webjs/routes.d.ts route union.
  • docs/app/docs/metadata-routes/page.ts: annotate the metadata example with Metadata, add a JSON-LD (jsonLd) subsection, and add metadata.preconnect / metadata.dnsPrefetch (plus the auto-vendor preconnect note).
• Source of truth to copy from: agent-docs/metadata.md (around L17 for the type, L198 to L231 for preconnect, L235 for JSON-LD), agent-docs/typescript.md (around L190 for the type, L215 for props, L252 for the route union), agent-docs/advanced.md (around L187 to L210 for preconnect).
• Note: the ?v= asset-hash half of Content-hash asset URLs for immutable cache, add preconnect hints #243 is already documented on the no-build and deployment pages; only the preconnect half is missing.

Acceptance criteria

• The typescript docs page documents the Metadata type, PageProps / LayoutProps, webjs types, and the route union.
• The metadata-routes docs page annotates metadata with Metadata, documents jsonLd, and documents preconnect / dnsPrefetch.