Doxygen: more complete and useful config

The results of this config are available at https://xonotic.org/doxygen and the logs are there too.
The Doxyfile included here is loaded by special case into the script, I'll remove that once master branch has the updated one.

The main issue with the old config is not including all the source code, for example:
old: http://timepath.github.io/scratchspace/d1/dc9/hlac_8qc.html
new: https://xonotic.org/doxygen/qcsrc/hlac_8qc.html

Several smaller improvements are also included.

This does increase file size and might cause the pipeline's doxygen stage to fail.
We don't need to use gitlab's resources for this and it's more efficient to run doxygen on srv04 (no uploading or installing the same packages every time), so I disabled the pipeline's doxygen stage.

Doxygen builds run at low priority so it won't interfere with interactive services.
The docs are updated automatically when gitlab sends the webhook event indicating there was a push to master, which takes less than 2 minutes.