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.
The widget won't work if these parameters are not set correctly so it seems reasonable to me to validate them & notify of mistakes rather than failing silently if someone gets them wrong in the future.