feat: migrate docs to Fumadocs framework

- Auto-generated sidebar from file structure
- Built-in search and TOC
- MDX native support with frontmatter
- Removed hand-written Sidebar component
- Docs now at /docs/* route (Landing/Blocks/Create/Examples unchanged)
- Content in content/docs/ as MDX files

packages/docs/components/Header.tsx

@@ -3,9 +3,9 @@ import Link from 'next/link';
 import { usePathname } from 'next/navigation';
 
 const navItems = [
-  { label: 'Docs', href: '/grid-system' },
-  { label: 'Components', href: '/components/article' },
-  { label: 'Themes', href: '/theme' },
+  { label: 'Docs', href: '/docs/grid-system' },
+  { label: 'Components', href: '/docs/components/article' },
+  { label: 'Themes', href: '/docs/theme' },
   { label: 'Blocks', href: '/blocks' },
   { label: 'Create', href: '/create' },
 ];