docs: enable warning as errors

I noticed in the docs build in noxfile.py, warnings are not treated as errors because the `-W` option is missing here:

https://github.com/googleapis/python-aiplatform/blob/cb904d772abd0836f67b1034074395ce7a032d66/noxfile.py#L279-L280

https://github.com/googleapis/python-aiplatform/blob/cb904d772abd0836f67b1034074395ce7a032d66/noxfile.py#L291-L301

```
-W
Turn warnings into errors. This means that the build stops at the first warning and sphinx-build exits with exit status 1.
```

https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-W

There is a workaround in owlbot.py to only fail the docs build if there are errors.

https://github.com/googleapis/python-aiplatform/blob/cb904d772abd0836f67b1034074395ce7a032d66/owlbot.py#L148-L149

The docs build is a form of static analysis and can catch docs issues which are not caught upstream. These issues may be reported via warnings. I checked the history and it looks like warnings as errors was disabled in https://github.com/googleapis/python-aiplatform/pull/22.

To enable warnings locally, make the following change
```
(py39) partheniou@partheniou-vm-3:~/git/python-aiplatform$ git diff
diff --git a/noxfile.py b/noxfile.py
index f90f5cad8..f134192e8 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -290,6 +290,7 @@ def docs(session):
shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
session.run(
"sphinx-build",
+ "-W",
"-T", # show full traceback on exception
"-N", # no colors
"-b",
```

The first warning which appeared is
```
Warning, treated as error:
/usr/local/google/home/partheniou/git/python-aiplatform/google/cloud/aiplatform/v1/schema/trainingjob/definition_v1/types/automl_image_classification.py:docstring of google.cloud.aiplatform.v1.schema.trainingjob.definition_v1.types.AutoMlImageClassification.inputs:1:duplicate object description of google.cloud.aiplatform.v1.schema.trainingjob.definition_v1.types.AutoMlImageClassification.inputs, other instance in aiplatform/definition_v1, use :noindex: for one of them
```

Start in noxfile.py at the docs session and compare the sphinx-build invocations at the linked lines; the issue identifies the missing -W option. Run the docs nox session and inspect the reported duplicate object description in the generated documentation. Done means the docs build treats warnings as errors and the reported warning is addressed.

索引モデルが issue の本文から書いたものです。