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 values are hardcoded and passed into this widget without any user input. As a developer the comprehensive documentation at the top or in the component docs provides enough information without having to add checks that will also always execute in production. This would make more sense if the data was variable and not defined by a developer that tests the code extensively.