feat: reposition BitRouter and simplify documentation

[SPIKESPIGEL404]

Summary

• reposition the landing page around token-minimizing workflows and the open-source product
• unify the global header, fold enterprise into pricing, and refresh comparison and CTA content
• reorganize documentation into Overview, Usage, Configuration, Customization, Reference, and Development
• keep Changelog top-level and restore the native Fumadocs sidebar presentation
• refresh generated CLI, models, API reference metadata, redirects, and LLM documentation exports

Validation

• pnpm test — 130 tests passed
• pnpm lint:docs — 44 docs passed
• pnpm build — 274 static pages generated
• git diff --check
• browser-checked Docs and Changelog layouts with no console errors