chore: reorganize docs apps (#7228)

* reorganize docs apps

* add README

* fix directory

* add condition for old docs

www/apps/book/mdx-components.tsx

@@ -0,0 +1,17 @@
+import type { MDXComponents } from "mdx/types"
+import CustomMDXComponents from "./components/MDXComponents"
+
+// This file allows you to provide custom React components
+// to be used in MDX files. You can import and use any
+// React component you want, including components from
+// other libraries.
+
+// This file is required to use MDX in `app` directory.
+export function useMDXComponents(components: MDXComponents): MDXComponents {
+  return {
+    // Allows customizing built-in components, e.g. to add styling.
+    // h1: ({ children }) => <h1 style={{ fontSize: "100px" }}>{children}</h1>,
+    ...components,
+    ...CustomMDXComponents,
+  }
+}