Add docs for `fee-recipient-address`

🎯 Problem to be solved

We need to add documentation on how Charon handles the fee-recipient-address for block proposals as it is not trivial. And it can be confusing for operators as they can get confused whether to set fee-recipient-address on validator client or not.

🛠️ Proposed solution

Add docs describing how charon handles fee-recipient-address. The documentation can include:

• Charon sets fee recipient address based on what is there in cluster-lock.json which is generated at the end of DKG ceremony: https://github.com/ObolNetwork/charon/blob/db040d559dae37b44f5c24ea6534ce4946cb6482/app/app.go#L911-L953
• It sets fee recipient address by periodically calling prepareBeaconProposer beacon API: https://ethereum.github.io/beacon-APIs/#/Validator/prepareBeaconProposer
• Charon swallows this API call if called by a validator client.

🧪 Tests

• Netlify deployment build tests passing

Review the linked app/app.go section and the Beacon API prepareBeaconProposer documentation first. Add operator-facing documentation covering cluster-lock.json, Charon's periodic API calls, and how calls from a validator client are handled; confirm the Netlify deployment build passes.