[Popover] provide more guidance and examples of using layering

[laurenmrice]

Acceptance criteria

• In the Usage tab guidance for Popover, we should include some more guidance and visual examples of how popovers are layered in different kinds of UIs.
• This could be for when a popover is triggered by a UI element and appears below the trigger, or when the popover has a tab tip and appears connected to the trigger.
• Some initial ideas of examples we can show, is when a popover is overlapping with a Page header and UI background, or overlapping different Cards and a UI background, etc.