Improve getting started documentation

[nawatts]

Some feedback on our current documentation: https://hail.zulipchat.com/#narrow/stream/123000-general/topic/Using.20Hail.20to.20download.20gnomAD.20data

[nawatts]

We assume the use of Dataproc. We could also provide an example for running locally with the GCS Connector.

[ch-kr]

we could add this to our gnomad email canned response (lumping this question in with an explanation of Hail Tables being directories and not single files)