Add side floating navigation to the content (about) pages

This PR implements the design for https://bugs.caret.cam.ac.uk/browse/PAVO-111. A side navigation is added to each content page which scrolls with the page to stay in view.
Could do, but then I'd have to override the padding as well as the border in the .active and :hover states, both for the top level and second level of the nav.

I feel that a transparent border on the idle state is a better fit for what's conceptually happening in the nav.