Move CSS files to includes

Concatenating the 4 CSS files will speed up the docs. It also does a little minification with built-in liquid filters.

The `404.html` (#1475) can now easily include inline CSS, according to [Github Error Page Styleguide](https://github.com/styleguide/templates/2.0).

~~And I’ve updated `normalize.css` to 2.1.3.~~