DOC: Update communication channels in documentation

[VeckoTheGecko]

Is your feature request related to a problem?

With #11246 , we mention in our contributing docs the following ways to ask for help:

• GitHub Discussions (https://github.com/pydata/xarray/discussions )
• Xarray channel in the OSSci Zulip ( https://ossci.zulipchat.com/#narrow/channel/582428-Xarray )
• Discord (https://discord.com/invite/wEKPCt4PDu ) - dormant
• mailing list (https://groups.google.com/g/xarray ) - dormant
• "python-xarray" tag on Stack Overflow ( https://stackoverflow.com/questions/tagged/python-xarray )

Describe the solution you'd like

I want to reduce the fragmentation of our help/communication channels. Zulip replaces the need for the Discord server (especially now we can easily talk with the Zarr people). Also, I don't think the mailing list serves a significant purpose now that we have Zulip (though thats perhaps an opinionated take)

I propose that we retire the Discord and the mailing list by doing the following:

• Update our core team guide and contributing docs
  • Mention that our primary channels to have loose discussion are GitHub discussions and the Zulip channel
  • We still monitor questions on StackOverflow (? I assume since the text was already there that someone does?)
  • Mention that we have previously had discussions on the mailing list and Discord but they are no longer used (in case people want to find prior communications)
  • Update the help-diagram flowchart
• Email the mailing list https://groups.google.com/g/xarray , and message the Discord mentioning that we're now on Zulip
• (optional) update the Discord permissions so nobody can send messages in it
  • looking at the chat history, I think @TomNicholas has admin perms - maybe you want to give that to me as well (username @aballofmoss) and I can handle any updates we want to make

Describe alternatives you've considered

No response

Additional context

No response

[VeckoTheGecko]

cc @pydata/xarray

[shoyer]

This sounds great to me. Thanks, Nick!

To be honest, I'm not sure anyone monitors Stack Overflow anymore. I used to do this in the very early days of Xarray, but it's been years now.