[Website] Add Zulip details to the Communication page

Zulip is currently used for real time development/technical discussions among many Arrow developers. We should add the channel and sign-in details to the https://arrow.apache.org/community/ page. Some guidance around the use of Zulip versus the mailing list would be helpful as well.

To sign in, go to https://ursalabs.zulipchat.com and make an account.