Document how to import custom Primer CSS

[shawnbot]

We need some "official" way to import Primer CSS. Here's how we've done it in our docs site:

1. At first, we just linked out to a bunch of unpkg.com URLs. This is okay for demos and sandboxes, but not good for production.
2. In Make static assets available on path alias #238, I set up the Next sass plugin to output a static CSS file as part of the webpack build. This is okay too, but we need to address other use cases.

[shawnbot]

Maybe the <StyleProvider> introduced in #158 should be responsible for outputting the <link> tags as well? And maybe it could take a baseURL just in case folks aren't comfortable with linking to unpkg.com?

[shawnbot]

Okay, after hacking on this in #238 so that we can use Next's sass loader, I have some better-formed opinions on how this should be done. Here's a proposed plan:

1. Introduce a <BaseCSS /> component that includes primer-base and whichever other styles we need to support existing components.
2. As we factor out Primer CSS from components, we remove the imports and npm dependencies. (For instance, primer-labels goes away when we
3. When we're done refactoring components, all that should remain is primer-base.
4. Figure out how to implement style encapsulation correctly.
5. Once styles are properly encapsulated and <BaseCSS /> is no longer needed, we make that component render null.
6. Remove the <BaseCSS /> component in the next major release.

What do you think, @broccolini @emplums @jonrohan?

[emplums]

Curious what you were thinking for Figure out how to implement style encapsulation correctly. ? Does "styles are properly encapsulated" === using all CSS in JS or are you thinking about something else?

[shawnbot]

@emplums I honestly don't know. Like, should all our components have the default font-family and line-height? I don't know how total style encapsulation is really possible without doing that. Or do we add a component that inlines the base styles with <style scoped>? /cc @broccolini

[shawnbot]

Done in #260