Created by: jhuashao
This pull request marks the addition of Progressive Disclosure Design Guidelines into primer/design.
Preview of Progressive Disclosure Guidelines on primer/design: https://primer-design-progressive-disclosure-guidelines.now.sh/design/ui-patterns/progressive-disclosure
Progressive Disclosure Initial Draft Issue: https://github.com/github/design-systems/issues/547
Tracking Issue: https://github.com/github/design-systems/issues/546