Fix position of headings when following links

[MattMS]

The navbar will overlap the content when following a link such as http://ramdajs.com/#partial-builds

This was mentioned by @davidchambers in ramda/ramda#2025.

[MattMS]

We cannot use the hack fix from #161 because the HTML is generated from Markdown.

Using the body {padding-top: @navbar-height;} recommended fix does not seem to work either.

[davidchambers]

What I've done in the past is given each heading top padding and negative top margin.

[MattMS]

That seems to work, but replacing the existing top margin and padding changes the layout.
So margins that would normally collapse together are no longer able to.

I've got the code written and working so I'll open a PR for further comments.