Only report navigation <link> elements outside of head #306

Related to w3c/wcag#2297

The procedure is to

Check that all link elements pertaining to navigation occur in the head section of the document.

But I can see that we're checking all link, regardless of their relation to navigation, including rel="stylesheet" which doesn't make any sense.

Please, consider excluding rel="stylesheet" or only including rels related to navigation, like Start, Next, Prev, Contents, Index, etc.