Describe operational procedure during Vitess reshard

We need to elaborate on the meaning of the connector being up and running when the reshard operation takes place.

Basically, Reshard operation has multiple manual steps (todo: list the steps in the documentation), of which the last step is deleting the old shards. The connector is allowed to shut down or to crash and will continue from the correct offset, before the last reshard operation step (deleting the old shards).