chore: reorganize docs apps (#7228)

* reorganize docs apps

* add README

* fix directory

* add condition for old docs

www/apps/book/components/MDXContent/index.tsx

@@ -0,0 +1,12 @@
+import { MDXRemote } from "next-mdx-remote/rsc"
+import MDXComponents from "../MDXComponents"
+
+type MDXContentProps = {
+  children: string
+}
+
+const MDXContent = ({ children }: MDXContentProps) => {
+  return <MDXRemote source={children} components={MDXComponents} />
+}
+
+export default MDXContent