Simplify Doxygen generation

Currently CMake calls a script that does nothing but call Doxygen,
something CMake could happily do itself. And we have two levels of
subfolders for holding Doxygen-related files, with a piece of outdated
unrelated (and insufficiently-helpful) DGI documentation as a bonus.

Let's instead use ONE directory for Doxygen, named doxygen, rather than
docs, since having two docs folders is confusing. As we can see by the
presence of documentation in the Doxygen folder. Remove this extra
file. (It could be moved to Docs instead, but honestly does not look
worth keeping.)