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/owlbot.py b/owlbot.py
index f692f70..fa03d69 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -7,11 +7,39 @@
 # Add templated files
 # ----------------------------------------------------------------------------
 templated_files = common.py_library(unit_cov_level=100, cov_level=100)
+
+
 s.move(
     templated_files / ".kokoro",
     excludes=[
         "continuous/common.cfg",
+        "docs/common.cfg",
         "presubmit/common.cfg",
         "build.sh",
     ],
 )  # just move kokoro configs
+
+
+assert 1 == s.replace(
+    ".kokoro/docs/docs-presubmit.cfg",
+    'value: "docs docfx"',
+    'value: "docs"',
+)
+
+assert 1 == s.replace(
+    ".kokoro/docker/docs/Dockerfile",
+    """\
+CMD \["python3\.8"\]""",
+    """\
+# Install gcloud SDK
+RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] http://packages.cloud.google.com/apt cloud-sdk main" | \\
+    tee -a /etc/apt/sources.list.d/google-cloud-sdk.list \\
+  && curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | \\
+    apt-key --keyring /usr/share/keyrings/cloud.google.gpg  add - \\
+  && apt-get update -y \\
+  && apt-get install python2 google-cloud-sdk -y
+
+CMD ["python3.8"]""",
+)
+
+s.shell.run(["nox", "-s", "blacken"], hide_output=False)