[doc] Improve doxygen documentation

Carsten Gräserrequested to merge
doc/modernize-doxygen into master
  • Improve structure of mainpage
  • Move file defining doxygen groups to .hh-file since doxygen always adds a top level entry for .md-file which is undesied here, because the content is in the auto-generated Topics page (formerly know as Modules).
  • Add doxygen page with licenses.
  • Use doxygen-awesome.css which provides many small improvements (font sizes, spacing, ...) and an overall more lightweight modern look.

Merge request reports