Consider add instruction for switch executor to LocalExecutor when using postgresql

Airflow use SequentialExecutor by default. However SequentialExecutor does not support postgresql, so it could be better to mentioned this when user decide to use postgresql instead of sqlite.

https://airflow.apache.org/docs/apache-airflow/stable/executor/sequential.html

Start by locating the setup or executor documentation for choosing an executor and review the linked SequentialExecutor page. Update the relevant documentation to explain the PostgreSQL limitation and instruct users to choose LocalExecutor instead; done means the PostgreSQL setup guidance is explicit.

Written by the indexing model from the issue text.