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/source.config.mjs

@@ -0,0 +1,11 @@
+// source.config.ts
+import { defineDocs, defineConfig } from "fumadocs-mdx/config";
+var { docs, meta } = defineDocs({
+  dir: "content/docs"
+});
+var source_config_default = defineConfig();
+export {
+  source_config_default as default,
+  docs,
+  meta
+};