Negative-margin heading links covering above content

[jacobmischka]

Negative margins on headings are (unfortunately) probably the best way to fix the internal-links-with-a-fixed-navbar problem, but right now the link is covering the content above it. Usually this doesn't make a difference, but sometimes it does (like in the scrollbar introduced in #793, not that one should usually encounter the issue on a device with a pointer, but still).

I think the heading should have the negative margin, but not the anchor tag.

Thanks!