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.
I'm not convinced this improves readability. We only use these once so it makes more sense to not define them up front or, if you really want to, only define them in the function where you'd be using them.