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.
5 files changed