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.
It's just overriding the style in `oae.base.css` which only specifies opacity: https://github.com/CUL-DigitalServices/avocet-ui/blob/0.0.6/shared/oae/css/oae.base.css#L85