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/source.config.ts

@@ -0,0 +1,7 @@
+import { defineDocs, defineConfig } from 'fumadocs-mdx/config';
+
+export const { docs, meta } = defineDocs({
+  dir: 'content/docs',
+});
+
+export default defineConfig();