Guide about  API authentication (including custom)

**Description**

We should write a guide that describes how to add new ones or use the current authentication method.

More information about the docs for REST API is available:
[Docs for REST API](https://github.com/apache/airflow/issues/8143)

**Use case / motivation**

N/A

**Related Issues**

N/A