feat: add AI-friendly Markdown access

[Cc-Cece]

What changed

• add a documentation-wide Copy page action that exports rendered content as Markdown
• publish per-page Markdown only for the default Chinese documentation locale
• generate /llms.txt with links to all 50 Chinese documents
• publish /robots.txt with the existing sitemap location

Why

AI-assisted readers need a clean way to copy a complete document and fetch Markdown without navigating the rendered site. English HTML remains available normally, while AI-specific Markdown output is intentionally limited to Chinese.

Validation

• npm run typecheck
• npm run build
• verified 50 Chinese Markdown routes and 0 English Markdown routes
• verified /intro.md, /llms.txt, and /robots.txt return HTTP 200 locally
• verified /en/intro.md returns HTTP 404 locally
• reviewed rendered Markdown from custom MDX components

npm run check:docs still stops on four UTF-8 BOM files already present on origin/main; its frontmatter step passes, and the build/typecheck steps were run separately.

[Cc-Cece]

Reviewed the full remote diff and generated artifacts. Scope is limited to the copy-page integration, Chinese-only Markdown/llms output, and crawler metadata. TypeScript, local production build, route checks, and GitHub Actions all pass. No blocking findings.