Add "utility-first" section to FE scss style guide

What does this MR do?

This MR:

• Cleans up the readability of the "Utility Classes" section in the style_guide_scss.
• Adds a section to promote a "utility-first" approach.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Links

• Closes gitlab-ui#38 (closed)