Add instructions to run a local Airflow server

[will-cromar]

Description

See README diff

Tests

Tested instructions locally with a WIP DAG.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[RissyRan]

Thanks Will! How will this feature look like?

I ran this standalone before, and recall I need to do some setup. Is this the same for you? Could you provide one simple example for the run? I think this will be helpful for users as well.

[will-cromar]

Thanks Will! How will this feature look like?

I ran this standalone before, and recall I need to do some setup. Is this the same for you? Could you provide one simple example for the run? I think this will be helpful for users as well.

I actually didn't have to do any other setup besides authenticating with GCP gcloud auth login --update-adc . airflow standalone does the one-time setup and prints a link to a local server. I didn't try all of the DAGs, so I don't expect them all to work. Some might be coupled to the composer environment, or people might not have the right permissions (e.g. to directly create QR in the project).

I'm contributing this as a quick hack that may or may not work, but we should expand it over time to work with more task types.

[RissyRan]

Approved with a nit.