Add How-To guide for PostgresOperator

[drdenzy]

It explores how to create tasks that interact with the PostgreSQL database.

This How-To guide demonstrates some of the best practices for using the PostgresOperator class.

closes: #11917

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[potiuk]

Really nice - few cosmetic coments, One question - is it possible to extract out some of the code examples and put it together as a fully-fetured example dag? We do that in other operators (look at the Google Howto Guides). This has the benefit that people can see a complete example in the example_dag as well as relevant snippets of code in the guide and navigate between them.

Also it would be great if such example dag is runnable as a System Test (https://github.com/apache/airflow/blob/master/TESTING.rst#airflow-system-tests) - we have not yet automated our system tests but we are planning to, It would be great to have such a runnable example_dag used here.

[github-actions]

The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[drdenzy]

Thanks @kaxil for the well thought out suggestions. I have made the necessary changes in line with your suggestions.