Added functionality to automatically ingest custom airflow.cfg file upon startup

[dominikhei]

closes: #35812

I have made the necessary changes to the docker-compose.yaml file as proposed within the issue. The relevant line is currently outcommented, but the documentation in the index.rst and comments above the outcommented line tell users
what to do. If one deletes the comment, an airflow.cfg file within ${AIRFLOW_PROJ_DIR:-.} is now added to /opt/airflow/airflow.cfgwithin the containers, once docker compose uphas been executed. The new line also should make it clear where the airflow.cfg is stored in the containers, as brought up by the issue.

As mentioned, I also added documentation for the changes in the index.rst file, such that users know how to automatically ingest their own airflow.cfg upon startup.

@potiuk Can you maybe have a look at this, since you also approved the original issue?

If I should change anything please let me know :)

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[potiuk]

Looks cool.

[Taragolis]

• Redundant whitespaces / newlines
• Missing whitespaces in the beginning of the comment (needs for pass statics checks)

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.