Improve Private Key backup instructions

[thomasheremans]

🎯 Problem to be solved

During the Lido Testnet #3, we received the feedback that there was some confusion as to what needs to be backed up during the cluster setup and DKG. Users were saving the actual ENR while they should be focussed on the private key instead from which an ENR can be generated.

🛠️ Proposed solution

Simpler would be to instruct people to backup .charon as this would align with them saving as well post DKG