Serve docs as Markdown when requested

[zahlekhan]

Summary

• detect when a docs request prefers Markdown using Fumadocs content negotiation
• rewrite matching /docs requests to the existing processed Markdown route
• preserve the normal HTML response for browser requests

Why

The docs already expose processed Markdown through the llms.mdx route, but clients requesting Accept: text/markdown still received the HTML page at the canonical docs URL. This adds the Fumadocs-recommended negotiation layer for AI agents and other Markdown clients.

Validation

• pnpm --filter @openuidev/docs types:check

closes #783