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/lib/source.ts

@@ -0,0 +1,8 @@
+import { docs, meta } from '../.source';
+import { loader } from 'fumadocs-core/source';
+import { createMDXSource } from 'fumadocs-mdx';
+
+export const source = loader({
+  baseUrl: '/docs',
+  source: createMDXSource(docs, meta),
+}) as ReturnType<typeof loader>;