Remove discord from the website

[oli-obk]

The official Rust discord was not used by team members anymore. Amusingly there are quite a few team members active on the "unofficial" community discord. Our discord is mainly a medium drain on mods as we get daily spam bots and weekly trolls.

After a quick check in with the council I am now moving to shutting down the discord. I have spread the knowledge about the incoming shutdown in the discord already. So our steps are

1. remove it from the website (this PR)
2. advertise it in the announcements channel on the discord
3. activate slow mode in all channels
4. lock all unused channels
5. wait for still-in-use channels to have been sufficiently used for migration discussions
6. lock all the channels and leave a final migration suggestion message

I am already in communication with the community discord owners and moderators, but we do not expect a massive influx of people, especially as we won't do a server wide ping for all the 60k ppl on the server, but instead just let those that still use the server realize that they should migrate.

We will not actually delete the server, as to allow people to keep linking to discussions on the server and preserve their history.

cc @rust-lang/leadership-council

[Eveeifyeve]

Should there also be a replacement link eventually to the unofficial one and note it's unofficial like https://nixos.org/community probably best to do it in another pr?

[oli-obk]

I have already reached out to the community discord maintainers and am planning to talk with other spaces so we can figure out sth like that. But since it's a first I don't just want to PR things but get the community involved