feat: modernize Jekyll site with professional design

- Completely redesign default layout with modern CSS
- Add Inter and JetBrains Mono fonts for better typography
- Rewrite index.md with proper HTML instead of mixed markdown
- Add feature cards with hover effects
- Improve table styling with shadows
- Better code block styling with dark theme
- Modern hero section with gradient background
- Remove Co-authored-by and attribution footers

Author: us