DOC: Add `Writers` group to Doxygen documentation
DOC: Add `Writers` group to Doxygen documentation

Add `Writers` group to Doxygen documentation.

The class `itk::QuadEdgeMeshScalarDataVTKPolyDataWriter` is documented
as belonging to such group.

hjmjohnson/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,thewtex/ITK,hjmjohnson/ITK,richardbeare/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,richardbeare/ITK,BRAINSia/ITK,BRAINSia/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,richardbeare/ITK,BRAINSia/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,Kitware/ITK,thewtex/ITK,richardbeare/ITK,BRAINSia/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,Kitware/ITK,thewtex/ITK,thewtex/ITK,hjmjohnson/ITK,thewtex/ITK,BRAINSia/ITK,richardbeare/ITK,richardbeare/ITK,Kitware/ITK,hjmjohnson/ITK