chore: omit 'docfx' from docs-presubmit build (#831)

We don't have a 'docfx' session in our noxfile (see #822).

Further tweak '.kokoro' configuration to get 'Kokoro docs-presubmit' running:

- Set up additional 'gfile_resource' in '.kokoro/docs/common.cfg'.
- Get 'gcloud' installed in '.kokoro/docker/docs/Dockerfile'
- Run 'docs' session with Python 3.8.

Closes #797.
diff --git a/noxfile.py b/noxfile.py
index d375b03..caeb272 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -142,7 +142,7 @@
     )
 
 
-@nox.session(python="3.7")
+@nox.session(python="3.8")
 def docs(session):
     """Build the docs for this library."""