Add note on node synchronization for BFT

[joaniefromtheblock]

The Besu docs do not seem to currently mention the use of snapshots or checkpoints for private networks. Include a clear note stating that if you are using Byzantine Fault Tolerance (BFT) consensus, you must set your node's synchronization mode to either FAST or FULL. Maybe it can be included on this page

See the following for more context:

• prevent startup if PoA and snap/checkpoint besu#6625
• allow snap/checkpoint with clique besu#6667