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 is only ever used once on a page as far as I can tell? It'd make more sense to move this inside of the function. That said, I'd also remove the `FOOTER_SELECTOR` and just construct the selector in the one place it's used.