Add documentation for shutting down an Ozone server

[ThisIsMissEm]

I asked about this on discord, and the consensus was that there wasn't official guidance on it, but that using:

npx @skywave/labeler clear

And checking with Label Scanner was the way to go.

(In our case we'll do that, but probably also delete that account entirely, as if we ever run a labeller or anything again, it'll be on two other domain names, instead of our .org)