Update how Doxygen documentation is generated

1) The Doxygen configuration files have been split out of src/main/c in
   favor of putting them in src/main/doxygen.

2) The target directory has been moved from doc to target/doxygen off
   the build root, so that our Jenkins job
   (http://jenkins.imagej.net/job/SLIM-curve) can publish the results.

3) The generated Doxygen files have been removed from the repository.
   Instead, we will rely on Jenkins to keep them up to date.

4) The user/dev configuration split has been removed, for now. The dev
   config was still empty. We can add it back later if needed, but
   personally I think anyone reading the Doxygen SLIM Curve docs is
   already in the "developer" camp and hence we can provide full detail
   in the one and only Doxygen documentation we produce.