[WIP] CSS section style/language overhaul

[hbillings]

This is an experiment to see if it's possible to rewrite some of our existing guidance in a way that goes out of date less easily. Also reorganizes the CSS section to be guidance and style guide sections, rather than calling the entire thing a style guide (which it isn't). This reorganization does mean that sometimes we talk about things in multiple places (i.e., the how is separate from the why, but they should link to each other), so we should talk as a group about how we feel about that.

Open questions:

• Should we still recommend KSS? It hasn't been updated in 3 years, and hasn't been significantly updated in nearly 5.
• Should we split the CSS documentation into recommendations and a style guide?
• What do we use for linting CSS these days? The Linting section recommends trying an experimental stylelint config that's not been updated in ages (we should have a guild convo about this). Meanwhile, sass-lint, while not deprecated, encourages people to use something else if at all possible. And Sass itself is deprecating its Ruby implementation in favor of a Dart one this March.
• Related: Do we have a linter that works for non-Sass projects?