Improve classes and modules docstring

### Body I have observed that the Python API documentation on the Airflow website lacks information about deprecation and the corresponding replacement modules or classes. In many cases, users are only alerted about these changes through log warnings. The purpose of this task is to enhance the docstrings, focusing particularly on utilizing the Sphinx cross-referencing syntax to improve the documentation. For example: Not documented deprecation: https://airflow.apache.org/docs/apache-airflow-providers-cncf-kubernetes/stable/_api/airflow/providers/cncf/kubernetes/operators/kubernetes_pod/index.html Documented deprecation: https://airflow.apache.org/docs/apache-airflow-providers-slack/stable/_api/airflow/providers/slack/notifications/slack_notifier/index.html Add deprecation docstring for: - [ ] Airflow core modules and classes - [x] Airflow providers modules and classes - [ ] Airflow core methods - [ ] Airflow providers methods ### Committer - [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.