Update UI Guide with SVG guidelines

This addition to the guide is to provide some guidelines to UX designers when exporting SVGs. Please let me know if anything is unclear or if you any improvements so we can document it clearly for everyone.

cc / @hazelyang @cperessini @dimitrieh @connorshea @annabeldunstone @dzaporozhets 

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ee/issues/872

See merge request !5748