Docs on how to release mercury

Let's leave here `verify` or `staging` only. The `sandbox-mercury` is an internal machine and we could save its name for ourselves ;)

Also, I'd leave here only one of those three possibilities and add note below that there are other environments which we can deploy stuff to. The `a/b/c` notation is clear for us but maybe for someone else `a|b|c` would be better so at the end I'd go with one "example" and a note about other possibilities below. What do you think about it? :space_invader: