doc.qt.io: Don't obscure page content when scrolling up

Normally, when users scroll up they want to reveal more content above the screen. However, the current behaviour of the auto-hiding header causes the content to become even more obscured than before (doc-scroll-up-obscure.gif)

The "compact" layout is better because scrolling up does not cause existing content to become obscured (doc-scroll-up-no-obscure.gif)