Proper mechanism for handling intra-page links

As seen in !216 (merged), the current style requires some ugly workaround in case there are internal links on a given page

Start of discussion is here

Proposed design for a solution is here

• Provide scroll-padding-top: in main.css
• Overload the rule for pages that have particular layout (API for example)
• Remove "anchor" CSS rule (in particular in publication page)
• Add @media rules for dealing with the different screen sizes