fix: fix docs build for generated clients

[parthea]

I've confirmed that the docs build now succeeds for generated clients with the changes in this PR:

(py39) partheniou@partheniou-vm-3:~/git/gapic-generator-python/tests/integration/goldens/credentials$ nox -s docs
/usr/local/google/home/partheniou/.pyenv/versions/py39/lib/python3.9/site-packages/setuptools/dist.py:945: SetuptoolsDeprecationWarning: The namespace_packages parameter is deprecated.
!!

        ********************************************************************************
        Please replace its usage with implicit namespaces (PEP 420).

        See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages for details.
        ********************************************************************************

!!
  ep.load()(self, ep.name, value)
nox > Running session docs
nox > Creating virtual environment (virtualenv) using python3.11 in .nox/docs
nox > python -m pip install -e .
nox > python -m pip install sphinx==7.0.1 alabaster recommonmark
nox > sphinx-build -W -T -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/
Running Sphinx v7.0.1
making output directory... done
[autosummary] generating autosummary for: credentials_v1/iam_credentials.rst, credentials_v1/services.rst, credentials_v1/types.rst, index.rst
loading intersphinx inventory from http://python.readthedocs.org/en/latest/objects.inv...
loading intersphinx inventory from https://gax-python.readthedocs.org/en/latest/objects.inv...
loading intersphinx inventory from https://google-auth.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory from https://gax-python.readthedocs.io/en/latest/objects.inv...
loading intersphinx inventory from https://googleapis.dev/python/google-api-core/latest/objects.inv...
loading intersphinx inventory from https://grpc.io/grpc/python/objects.inv...
loading intersphinx inventory from http://requests.kennethreitz.org/en/stable/objects.inv...
loading intersphinx inventory from https://proto-plus-python.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory from https://googleapis.dev/python/protobuf/latest/objects.inv...
intersphinx inventory has moved: http://requests.kennethreitz.org/en/stable/objects.inv -> https://requests.kennethreitz.org/en/stable/objects.inv
intersphinx inventory has moved: http://python.readthedocs.org/en/latest/objects.inv -> https://python.readthedocs.io/en/latest/objects.inv
intersphinx inventory has moved: https://grpc.io/grpc/python/objects.inv -> https://grpc.github.io/grpc/python/objects.inv
intersphinx inventory has moved: https://gax-python.readthedocs.org/en/latest/objects.inv -> https://gax-python.readthedocs.io/en/latest/objects.inv
building [mo]: targets for 0 po files that are out of date
writing output... 
building [html]: targets for 4 source files that are out of date
updating environment: [new config] 4 added, 0 changed, 0 removed
reading sources... [ 25%] credentials_v1/iam_credentials
reading sources... [ 50%] credentials_v1/services
reading sources... [ 75%] credentials_v1/types
reading sources... [100%] index

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [ 25%] credentials_v1/iam_credentials
writing output... [ 50%] credentials_v1/services
writing output... [ 75%] credentials_v1/types
writing output... [100%] index

generating indices... genindex py-modindex done
highlighting module code... [ 33%] google.iam.credentials_v1.services.iam_credentials.async_client
highlighting module code... [ 66%] google.iam.credentials_v1.services.iam_credentials.client
highlighting module code... [100%] google.iam.credentials_v1.types.common

writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded.

The HTML pages are in docs/_build/html.
nox > Session docs was successful.

Fixes #1714 🦕

[ohmayr]

Verified the changes locally and can confirm that this fixes docs build for generated clients.