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/mdx-components.tsx

@@ -1,7 +1,9 @@
 import type { MDXComponents } from 'mdx/types';
+import defaultMdxComponents from 'fumadocs-ui/mdx';
 
 export function useMDXComponents(components: MDXComponents): MDXComponents {
   return {
+    ...defaultMdxComponents,
     ...components,
   };
 }