docs: document patterned templates (RFC 823)

[ubugeeei]

Description of Problem

RFC 823 needs documentation for its patterned-template reference implementation.

Proposed Solution

Add a guide beside conditional rendering and API entries for v-match and v-when. Explain patterns, branch-local bindings, rest, guards, narrowing, exhaustiveness errors and unreachable-arm warnings, with examples.

The guide and API entries identify the syntax as a Draft reference implementation. They follow the current RFC and leave its property-presence question open. Companion PRs: core #15531 and language-tools #6207.

The basic SFC example puts v-match directly on the top-level <template>. Explain its equivalence to an inner match and retain the same arm scope and coverage rules. The reference notice identifies the current inline-HTML and bundler integration limits.

Additional Information

pnpm build passed. Browser checks at 1440 px and 390 px found no horizontal overflow or page errors. The Netlify preview checks passed.

pnpm type fails on the unchanged .vitepress/config.ts:795 Plugin tuple cast. This PR does not modify that configuration.

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[ubugeeei]

Draft reference documentation for RFC 823, paired with core #15531 and language-tools #6207.

At bad77b085, Netlify preview checks pass. The basic example uses v-match on the SFC top-level template and explains its equivalence to an inner match. Arm scopes, shadowing, duplicate declarations, narrowing and exhaustive coverage are unchanged.

The reference notice identifies the current inline-HTML and bundler integration limits. The existing pnpm type failure in .vitepress/config.ts:795 remains recorded in the PR description.