[node] Add docs on node key set up

[PierreBesson]

For bootnodes and now also for validators (paritytech/polkadot-sdk#3852), it is necessary to set up a network key, this should be reflected in our readme docs with examples on how to set it up:

• node.customNodeKey, then add the key generated with polkadot key generate-node-key
• Point existing node key k8s secret with node.existingSecrets.nodeKey
• or set node.persistGeneratedNodeKey=true, this will generate a key on startup and store it to the volume