[GA-1502] Style guide

[kafitzgerald]

A style guide was suggested here by @jukent. I think this is probably a good idea and didn't want it to get lost so logging it here.

I think we can likely "outsource" a good bit of this (i.e. point to external resources) and codify some of it in tooling, but it'd be nice to have it documented.

Given the nature of this resource, we'll probably want to think a bit more broadly than code style as well e.g. citations and other writing related items and also accessibility and inclusive language.

This Write the Docs Style Guides resource might be helpful.