Move discussion from ASF Slack to discord

[alamb]

Background

DataFusion currently has both a discord (open sign up) and slack channel (invite required); https://arrow.apache.org/datafusion/contributor-guide/communication.html#slack-and-discord

While we issue invites to the slack channel to anyone who desires, this sometimes takes several days (like @fernandocast for example #7249 (comment))

Problem

I think requiring people to get an invite places an unnecessary burden on the DataFusion community -- both for existing contributors to make the slack ASF invite (it takes a back and forth to get the email) as well as for new contributors or users to find the slack conversation

There have been previous discussions about this on the mailing list and one concern is that discord communication is not archived / public the same way that the ASF handles slack backups. However, I think since the communication is still open but much more ephemeral we don't have to treat it the same as issues and the mailing list

Proposal

I propose we move all discussion to the Discord channel and discontinue use of the slack channel.

We would, of course, continue to keep all communications open and move anything of substance into tickets and on the main dev@arrow and not make decisions on Discord.

I believe there is precident for this as the Apache rrow mono repo has a zulip chat (see references to): https://lists.apache.org/[email protected]:2023-7:zulip