restores parmed intersphinx (#2978, Issue #2951)

Fixes issue #2951 

## Work done in this PR

- Restores the parmed intersphinx link.

package/doc/sphinx/source/conf.py

@@ -344,7 +344,7 @@
                        'https://networkx.github.io/documentation/stable/': None,
                        'https://www.mdanalysis.org/GridDataFormats/': None,
                        'https://gsd.readthedocs.io/en/stable/': None,
-                    #    'https://parmed.github.io/ParmEd/html/': None,
+                       'https://parmed.github.io/ParmEd/html/': None,
                        'https://docs.h5py.org/en/stable': None,
                        'https://www.rdkit.org/docs/': None,
                        }