Add scroll margin to anchor targets that need it

[rogermparent]

scroll-pt-top on the body makes up for the sticky header exactly, and elements that want to have a little extra space can set scroll-mt to be used in addition.

Nice addition. I think we can add them to other linkers as well like details link, args link etc which also provide hash links beside headers.

Originally posted by @yathomasi in #140 (comment)

That PR added scroll-margin to docs headings, meaning that when navigated to via anchor (e.g. href="#heading-id") the docs headings will have some extra space above them instead of hugging the navbar. This Issue tracks adding scroll-margin to other elements throughout the site that would benefit the same way.