docs: explain AI-friendly access

[Cc-Cece]

What changed

• add a compact AI access section after the project cards on both documentation homepages
• explain the Chinese llms.txt index and per-page Markdown route convention
• add a one-click localized prompt for AI tools
• provide copied/failed feedback, keyboard focus styling, and responsive layout

Why

The site now publishes AI-friendly Markdown, but visitors had no concise explanation of how an AI tool should discover and consume it.

Validation

• npm run typecheck
• npm run build
• verified Chinese and English homepage content in generated HTML
• verified /llms.txt and /intro.md return HTTP 200 locally
• reviewed React event handling, timer cleanup, semantic button usage, focus styling, and live feedback

The existing two broken-anchor warnings are unchanged from main. The in-app browser was unavailable, so system clipboard contents could not be asserted automatically; the clipboard handler and rendered controls were reviewed directly.

[Cc-Cece]

Reviewed the complete remote diff. The AI access section is placed after the project entries, keeps the explanation concise, links directly to llms.txt, and supplies localized prompts with accessible copy feedback. React event handling and timer cleanup are sound. TypeScript, production build, local HTTP checks, and GitHub Actions pass. No blocking findings.