How to import CSS from an npm package?

The official ember-cli docs, and [this ember atlas page](https://www.notion.so/importing-CSS-from-a-npm-package-c15546025ce44adfbeb4907eab118e22)

say to use `app.import`.

We don't want to encourage any usage of app.import, as it's archaic AF -- _so_, I think we should document how to use css `@import` in the `app.css`.

I think with embroider and ember-auto-import, it should "just work", but it'll be good to document, as we have no examples _anywhere_.