Lint/format docs

[oliverjam]

There's currently a git pre-commit hook that auto-formats all files with Prettier. However this relies on contributors running npm install at least once, which isn't necessarily going to happen.

We should have a GitHub Action to automatically run this against any PR. It should also use remark-lint to check the Markdown files for common mistakes, inclusive language, plus any other stylistic choices we want to enforce.

https://katydecorah.com/code/lint-markdown/
https://katydecorah.com/code/copyeditor/