Account for header bar in scroll target offsets

[mootari]

Prevents sticky headings from covering framework names when navigating to a component via the sidebar.

Before	After

[mootari]

It may also help to show when content is covered by the sticky headings, by applying the same blur filter that's already used for the Header component. Unfortunately that may be more involved since the lack of a left padding would cause the blur effect to just cut off.