Amazon Sagemaker Sample DAG and docs update

[ferruzzi]

Cleans up the existing sample DAG and docs for the SageMaker operators.
Part of a project to standardize AWS sample dags and docs in preparation for adding System Testing.

Related: #21523
Related: #21475
Related: #21828
Related: #21920
etc...

[ferruzzi]

Please note: The operators have do_xcom_push=False because they currently fail at the very end with the message "Object of type datetime is not JSON serializable". This is a temporary workaround and can be fixed by adding json.dumps() to the return statements. For example: change return self.hook.describe_endpoint_config(endpoint_info['EndpointConfigName']) to return json.dumps(self.hook.describe_endpoint_config(endpoint_info['EndpointConfigName']), cls=AirflowJsonEncoder).

I will submit a fix for those next, but that is outside the scope of this PR.

[ferruzzi]

@eladkal may want eyes on this one since he has reviewed most/all of the others in this collection of changes.