Add CoC and deescalation docs for Zulip

[infinisil]

We worked out reasonable CoC (primary author @endocrimes) and deescalation docs (primary author @joepie91) together for the Zulip instance. Let's get this merged so we can link to it.

Feel free to add your signature with :+1:!

Note that we're discussing this live in the Zulip setup coordination Matrix room.

[infinisil]

As a proposed Zulip moderator I pledge to act reasonably and in the best interest for establishing a great community-based governance!

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nixos-foundation-board-giving-power-to-the-community/44552/119

[endocrimes]

As a (new) member of the moderation team I also pledge to act reasonably and in the best interest of establishing a great community-based governance.

[RaitoBezarius]

As a proposed Zulip moderator, I pledge to act reasonably and in the best interest for establishing a great community-based governance.