Docs examples 09/14: Rework modal and drawer examples

Part 9 of 14 in the stack that splits up the approved draft !6295 (closed) into reviewable pieces. The content is exactly what was reviewed there; I've only sliced it.

Related to #3575.

Pages: modal, drawer. Realistic content (set status, terms of service, keyboard shortcuts, destructive delete-project confirmation), a new Behavior > Scrolling subsection collecting the scrolling guidance with its example, and a Code reference "Hiding the footer" section for the hide-footer example.

Safe to publish without the MRs after it.