style(docs): redesign Git Pages with GitHub-style theme

- Update color system to GitHub style (#24292f, #d0d7de, #0d1117)
- Simplify Hero section (hide image, pure text + CTA)
- Add custom homepage components (home-header, feature-map, quick-start)
- Redesign English and Chinese homepage with feature card grid layout
- Add responsive design (3-col → 2-col → 1-col)

Ref: kimi-cli Git Pages design pattern

Author: LessUp