doc/Doxyfile: make docs reproducible & nicer
- Strip the source directory from path & include path. This removes non-reproducibility and make the doc shows correct include string.
- Exclude doc/qml/. There's not real code there, only QDoc docs. This removes bogus doc/qml/ in the file list. Unfortunately I cannot remove bogus doc/, because doc/ has to be included for Mainpage.md to be discovered. Listing doc/Mainpage.md only doesn't help either.
Alternative to and inspired by !14 (closed).