[1.5.X] Fixed broken links, round 3. refs #19516

Backport of b3a8c9dab8 from master

docs/conf.py

@@ -95,6 +95,7 @@ intersphinx_mapping = {
     'python': ('http://docs.python.org/2.7', None),
     'sphinx': ('http://sphinx.pocoo.org/', None),
     'six': ('http://packages.python.org/six/', None),
+    'simplejson': ('http://simplejson.readthedocs.org/en/latest/', None),
 }
 
 # Python's docs don't change every week.