Clarify FAB auth provider versioning and webserver_config.py deprecation for Airflow 3.x

[nrnavaneet]

Summary

This PR improves the FAB authentication documentation by clarifying the distinction between the apache-airflow-providers-fab version and the Airflow core version. It also updates the note to reflect that the webserver_config.py file is no longer used in Airflow 3.x and has been replaced by the auth_manager framework.

Updates

Added a .. note:: block to clarify:

• The FAB provider is versioned independently (currently at 2.3.0).
• It is compatible with Airflow 3.x, including 3.0.3.
• webserver_config.py is deprecated and replaced with auth_manager in airflow.cfg.
• Linked to the token authentication docs for more details.

Tests

• Rendered the documentation locally to verify correct formatting and clarity.
• Confirmed links and references display properly in the built docs.

Credits

• piosystems for raising the issue.
• Assisted by @chatgpt (OpenAI)

Closes #53573

[nrnavaneet]

Hi, Ive tried fixing ur issue by adding a note in the webserver-authentication file . In case u need any other fixes or changes kindly lmk :)