[doc] Use doxygen-awesome.css

There's a few annoying things with the default doxygen layout. In particular font sizes, text width and spacings are nasty. While all of this could be changed manually in the css, I don't want to investigate time on this. As a remedy we now use https://jothepro.github.io/doxygen-awesome-css which is a tuned css-file for doxygen fixing these issues and looking a little more modern (no more 90's style color gradients).

The css-file contains an MIT-licensed so including it is not a problem.