Establish CSS/ SCSS best practice

[hanbyul-here]

As we adopt USWDS, the team needs to write more CSS(including SCSS). But we do not have agreement on the best practice, such as how to name the class and when to use utility classes.

Acceptance Criteria

• Have a doc that goes through the best practice of CSS in the team

Items to include in the doc

• how to use selectors
• how to name classes
• how to name files
• when to use utility classes vs. when not to

[dzole0311]

I was jotting down some thoughts on this recently and collected them in this draft. It's still a draft and needs a sync with the team in order to collect more pain points and items to address: https://docs.google.com/document/d/1CySP1U3P2dElH8v-ruIWq_11E8FHpi-iZn_BSdNwcL4/edit?usp=sharing