geos-chem-shared-docs update: Update links & text to netcdf-scripts repo

This commit informs the GCHP superproject about the following
commits that were pushed to the geos-chem-shared-docs submodule

c73c80e RTD updates: Link to netcdf-scripts in netCDF supplemental guides
8990112 Now point to isCoards script at geoschem/netcdf-scripts
6125718 Update the "Work with netCDF files" supplemental guide

These updates now make sure that mentions to "isCoards" and "nc_chunk.pl"
now reference these files from the new netcdf-scripts repo at Github,
rather than from the NcdfUtil repo.  Also a link from coards-guide.rst
to the isCoards section of netcdf-guide.rst has been added

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>