Tracking issue for general scroll-dependent header behaviors as a scoped version of the general scroll-bound CSS feature.

Feature 1: Header hides/shows as user scrolls down/up

Feature 2: Inline sub-heading for page section docks to header when it is scrolled to the top of the viewport, and sticks there until user scrolls back up, or is replaced by another docking/sticky header, or is removed (user scrolls below relevant section)

/cc @aghassemi @cvializ