Improvements to documentation on slack availability to new users/developers

I know there's been some discussions about Slack, so if this overlaps or has been addressed/discussed please feel free to close it.

**Motivation**
Some OSS projects specify their method of communication (e.g., Gitter, Slack) directly on their README.md with a badge. While Slack is mentioned, it can often be buried. I think there's only one place it's mentioned and that's in the Getting Help page of the `astropy` docs. I just did a search for "slack" in `astropy/astropy` and no hits in any code. By having it front and center, it could help connect new contributors to developers and maintainers faster.

One other advantage that I see is to have specific Slack channels specified for each project (e.g., `astroquery`). This could help facilitate what seems to be a common occurrence that users ask questions in the `general` channel only to be directed to a specific channel.

Review the Getting Help page and README.md references mentioned in the issue, then search the repository for existing Slack guidance. Confirm the desired newcomer-facing locations and project-specific channels with maintainers before editing; done means the agreed locations and channel guidance are documented.

Written by the indexing model from the issue text.