feat(v2): allow passing remark, rehype, prismtheme to mdx-loader (#1543)

* feat(v2): allow passing remark, rehype, prismtheme to mdx-loader

* nits

website/docs/creating-pages.md

@@ -11,7 +11,7 @@ In this section, we will learn about creating ad-hoc pages in Docusaurus using R
 
 In the `pages` directory, create a file called `hello.js` with the following contents:
 
-```js
+```jsx
 import React from 'react';
 import Layout from '@theme/Layout';