Add network diagram(s) for private peers & clustering

[mDuo13]

The "Private Peers" doc or a related doc could use a network diagram showing how the private peer can be configured behind a firewall. For example it might show that a private-peer validator should be clustered with the public proxy server(s).

Private peers are possibly also expected to (usually) be used with clusters, so it might make sense to put them in the same diagram. Possibly two diagrams.

Collect details of the network architecture from relevant rippled experts like @mtrippled.