Migrate docs source to Fumadocs

[felixfeng33]

Summary

Migrate the docs app source pipeline from Contentlayer to Fumadocs MDX while preserving Plate's existing docs shell, registry-backed docs pages, raw markdown copy, and EN/CN route behavior.

What changed

• Add Fumadocs source config, generated-source aliasing, and Next MDX integration for apps/www.
• Replace Contentlayer route lookups with Fumadocs source access for English and Chinese docs routes.
• Split registry metadata helpers from runtime component lookup so source generation stays server-safe.
• Add explicit MDX client boundaries and compatibility fixes for package info, API lists, and accordions.
• Remove Contentlayer config/dependencies and add a docs source parity check wired into www typecheck.

Verification

• pnpm check
• pnpm --filter www check:docs
• pnpm --filter www exec next build
• Browser smoke previously covered /docs, /docs/installation, /docs/components/editor, /docs/table, /docs/api/core, /docs/releases, /cn/docs/table, and /docs/examples/slate-to-html.

Note: local registry generation was not run because repo rules forbid build:registry outside CI.

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Latest commit: 7443889

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