Doc: Update Public Interface as a result of Airflow 3.0

[kaxil]

https://airflow.apache.org/docs/apache-airflow/stable/public-airflow-interface.html is up for a refresh since 3.0

This will help cases like #51563

Users should consider airflow.sdk namespace as the Public Interface as a result of AIP072.

Reference:

• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#restricted-metadata-database-access
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#task-execution-api-task-sdk-aip-72
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#new-stable-dag-authoring-interface-airflow-sdk
• https://cwiki.apache.org/confluence/display/AIRFLOW/AIP-72+Task+Execution+Interface+aka+Task+SDK

We should suggest using official REST API or Python Client instead.

As a part of this task, we should scan other parts of the docs to be consistent with this messaging.

Related Issues/PRs:

• Docs setup for the Task SDK #51153
• Docs: Replace airflow.sdk.definitions to airflow.sdk for public facing docs #43010
• Create Task SDK docs #51283
• Airflow 3 Docs Meta Issue #49738