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.

Review the proposed routes/docs/:page/index.js structure and the referenced MDX runtime and Next MDX examples. Determine which MDX behavior or example this issue should specify, and define completion around an agreed route or rendering approach.

索引モデルが issue の本文から書いたものです。