docs: add enhanceApp placeholder

Author: ulivz

packages/docs/docs/.vuepress/enhanceApp.js

@@ -0,0 +1,8 @@
+export default ({
+  Vue, // the version of Vue being used in the VuePress app
+  options, // the options for the root Vue instance
+  router, // the router instance for the app
+  siteData // site metadata
+}) => {
+  // ...apply enhancements to the app
+}