Modern-looking Doxygen theme

Darwin uses a tuned Doxygen config, but the default Doxygen theme looks ancient and can improving this ourselves would be difficult. There are a few "modern" Doxygen themes around that could be used, including:

• https://jothepro.github.io/doxygen-awesome-css/index.html
• https://github.com/samvv/zen-doxygen-theme
• https://www.awalsh128.com/doxygen-themes/ (not that good imho)

Since the landing page of the docs will be the future user's first impression when coming from CAT, it should look shiny!