[doxygen] Very restrictive timeout
In cmake/scripts/RunDoxygen.cmake a timeout of just 5min is set. For larger modules and on slower machines (or if the process runs slower because of other stuff going on) this can easily be exceeded.
In !835 (merged) I set it to one hour. The question is do we need a timeout at all? There seems to be no way to overwrite the timeout in a downstream module. I just found out that for building the dumux docs for our website we had been patching dune-common to increase the timeout to not kill doxygen off too early.