Automatic generation of the documentation

[Lectem]

Possibilities are

• Publish to gh-pages from CI: https://github.com/Lectem/libmpo/blob/master/exportdoc.sh
• use readthedocs.io
  • requires breathe+sphinx setup
  • Need to use our own Doxyfile, rewrite the CMake doc target

[mosra]

Another possibility (sorry for the shameless self-promo):

• use the m.css Doxygen theme .. it's proven to work on Travis CI, even :)

[Lectem]

I might give it a try, but it implies doing the CI work first anyway :)

[Lectem]

Fixed by 7a2865e72ae400f7d627481efe5761c81fcc445e