docs: Update doxygen to have a more modern look
This adds the css from https://github.com/jothepro/doxygen-awesome-css to give our doxygen documentation a more modern feel.
N.B. The MIT licence is compatible with Apache 2, and copying the file is one of the recommended installation techniques.
Before:
After:
I've put the whole documentation here: https://emoyse.web.cern.ch/emoyse/doxygen (it's still copying at the time of writing, but should be there in an hour or so)
I'm opening this now, but I think it should be discussed in a meeting (ASCIG? SW Weekly?)