Update sharding document for public witness network

[haydentherapper]

Description

The public witness network is starting up, and we've already added the staging log to the test network. We need to specify that the log's public key should be added to the witness network once the key is created and before we spin the server up. Until the network is fully productionized and stable, we'll fail open, so it's technically fine to spin up the server before the witnesses are configured with the log's public key, but that will produce checkpoints without cosignatures.