docs(site): restructure GitHub Pages with new design system

- Rewrite index.html as full documentation portal with nav, hero, modules, roadmap
- Add guides.html as topic hub with staged learning path
- Create assets/site.css shared design system (glassmorphism, responsive)
- Update pages.yml workflow to include new assets and guides.html

Author: shijiashuai

.github/workflows/pages.yml

@@ -67,6 +67,8 @@ jobs:
         run: |
           mkdir -p docs/_site
           cp docs/index.html docs/_site/index.html
+          cp docs/guides.html docs/_site/guides.html
+          cp -r docs/assets docs/_site/assets
           cp -r docs/*.md docs/_site/
           # Copy Doxygen output
           if [ -d docs/api/html ]; then