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/index.d.ts

@@ -0,0 +1,3 @@
+import type { GetOutput } from "fumadocs-mdx/config"
+export declare const docs: GetOutput<typeof import("../source.config.ts").docs>
+export declare const meta: GetOutput<typeof import("../source.config.ts").meta>