W3C Homepage (+ other templates): move the "skip" link

[koalie]

Via internal mail from @bert-github:

The W3C home page is well designed in that respect [accessibility]. All the text makes
sense, all the sections are clearly labeled and in a logical order. The
only thing to improve could be to move the "skip" link (which allows to
scroll directly to the main content) to before the navigation menu,
because it currently isn't too useful.

[tripu]

(ie, move #w3c_crumbs before #w3c_sidenavigation)