Update Documentation

[emplums]

Our documentation is quite out of date now with all of our changes to the repository in the past few months! This PR takes a stab and updating them.

/cc @primer/ds-core

[emplums]

@shawnbot Do you have any documentation lying around from your team post/etc from the CSS restructure that we could use here: https://github.com/primer/css/blame/master/pages/css/getting-started/contributing.md#L174

slash, would you be up for updating these docs about the structure of Primer/CSS if you have a spare minute?

[shawnbot]

@emplums Pretty much all of the documentation for the restructure lives in either the RFC issue or the tracking issue. Let me know if you have a more specific question, though! After I wrap up some responsive issue work I can take a stab at updating some of this. Do you mind if I push to this branch while you're out?

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