Update css styleguide

Brings our CSS code style guide up to date.

Status: **Opened for visibility**

Reviewers: @kpeatt
Ticket: N/A

## Changes
- Adds info on our tooling and frameworks
- Adds background and attribution for our practices
- Details our variable naming convention
- Improves class naming section
- Adds section documenting conventions for styling states
- Misc fixes, e.g. broken links