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.
IMO It's a public API and it's inappropriate for a public API to accept erroneous input without reporting errors. Little annoys me more than things silently failing. Using these kind of checks consistently makes debugging a whole load load easier.