BUG: Fix documentation targets

Move 'doc' target to be created, initially with no dependencies, to the
root CMakeLists.txt, so that it is always available for non-Doxygen doc
targets (e.g. SlicerWizard-doc). Change doxygen doc target to be a
separate target that is then added as a dependency of 'doc' in the same
manner as SlicerWizard-doc. Remove logic to control when targets are
part of the default target except from the root 'doc' target (since
duplicating it is superfluous).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23004 3bd1e089-480b-0410-8dfb-8563597acbee