chore: reorganize docs apps (#7228)

* reorganize docs apps

* add README

* fix directory

* add condition for old docs

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

@@ -0,0 +1,9 @@
+import type { MDXComponents as MDXComponentsType } from "mdx/types"
+import { Link, MDXComponents as UiMdxComponents } from "docs-ui"
+
+const MDXComponents: MDXComponentsType = {
+  ...UiMdxComponents,
+  a: Link,
+}
+
+export default MDXComponents