geos-chem-shared-docs update: Add maintainer column to related-docs.rst

docs/source/geos-chem-shared-docs/supplemental-guides/related-docs
- Added a "Maintained by" column to the table "User manuals for
  GEOS-Chem and related software" to make it clear which packages
  are maintained by GCST and which are not.

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