Added ZODB to intersphinx

docs/conf.py

@@ -250,4 +250,8 @@ texinfo_documents = [
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+intersphinx_mapping = {
+    "python": ('http://docs.python.org/', None),
+    "persistent": ('https://persistent.readthedocs.io/en/latest/', None),
+    "ZODB": ("http://www.zodb.org/en/latest/", None),
+}