MDX example

No, `index.mdx` doesn't work out of the box.

But I think that's fine. Because `index.mdx` and `about.md` aren't routes.

I think it makes more sense to have:

- `routes/docs/:page/index.js`
- Something like: https://github.com/mdx-js/mdx/tree/master/packages/runtime

Or, recommend using Next:

- https://github.com/zeit/next-plugins/tree/master/packages/next-mdx

Why not just render it? Because the moment you want to customize providers, you need an escape hook.