GlDrawer: Define the behavior and usage guidelines for sticky footer in drawers

Problem

gitlab-org/gitlab-ui!2876 (merged) introduced an optional sticky footer property to the Drawer component. We need to define the footer usage and behavior guidelines.

Proposal

We need to answer such questions as:

• Should the actions in the drawer go in the header or footer, when and why?
• Should we use an overlap scrim above the sticky footer when content scrolls underneath it?