PubSub: Document how to choose the PubSub auth method

[plamut]

Closes #8137.

Describes a possible workaround for #7387, however, it does not close it, the PR just fulfills a GA requirement (documenting a possible workaround).

How to test

Steps to perform:

• Generate the pubsub docs locally (from inside the pubsub/ directory):

  $ nox -f noxfile.py -s docs
  

• Open the generated docs for authentication:

  $ google-chrome "file://$GOOG_CLOUD_PYTHON_DIR/pubsub/docs/_build/html/index.html#authentication"
  

Expected result:
The example is understandable and correct, the links point to correct locations.

Background

The issue itself is hard to reproduce in practice, but it was discovered in other PubSub clients that using JWT-based authentication avoids the issue. The goal of this PR is to document how different auth methods can be used with the Python client.

[xrmx]

Thanks @plamut! I've added a small comment as to me the key is the google-auth documentation and not the Credentials reference.

[plamut]

I marked the PR to close #8137 based on the issue reporter's feedback (thanks again, @xrmx ).

[anguillanneuf]

LGTM!