docs: generated message types reference proto-plus (#680)

Multiple issues have been filed indicating that json marshalling has
been broken and the fix is not obvious.
This will hopefully be ameliorated by indicating the inheritance from
proto.Message, with a link to the appropriate documentation.

gapic/ads-templates/scripts/fixup_%name_%version_keywords.py.j2

@@ -1,3 +1,4 @@
+#! /usr/bin/env python3
 {% extends '_base.py.j2' %}
 {% block content %}
 import argparse

gapic/templates/docs/%name_%version/types.rst.j2

@@ -3,3 +3,4 @@ Types for {{ api.naming.long_name }} {{ api.naming.version }} API
 
 .. automodule:: {{ api.naming.namespace|join('.')|lower }}.{{ api.naming.versioned_module_name }}.types
     :members:
+    :show-inheritance:

gapic/templates/docs/conf.py.j2

@@ -346,6 +346,7 @@ intersphinx_mapping = {
     "google.api_core": ("https://googleapis.dev/python/google-api-core/latest/", None),
     "grpc": ("https://grpc.io/grpc/python/", None),
     "requests": ("http://requests.kennethreitz.org/en/stable/", None),
+    "proto": ("https://proto-plus-python.readthedocs.io/en/stable", None),
 }
 
 

gapic/templates/scripts/fixup_%name_%version_keywords.py.j2

@@ -1,3 +1,4 @@
+#! /usr/bin/env python3
 {% extends '_base.py.j2' %}
 {% block content %}
 import argparse

noxfile.py

@@ -229,7 +229,7 @@ def showcase_mypy_alternative_templates(session):
     showcase_mypy(session, templates=ADS_TEMPLATES, other_opts=("old-naming",))
 
 
-@nox.session(python="3.6")
+@nox.session(python="3.8")
 def docs(session):
     """Build the docs."""