docs: recommend hardware wallet or multisig for production validator, not test keyring

[jcstein]

From Slack message:

1. The page to create a validator wallet should use --ledger and discuss --index, not the test backend. There's not much point for the only docs on running a validator not tell me how to run a production validator.
2. The page to create a validator wallet and the page to create a validator should use the same validator key monikey, not both validator and $VALIDATOR_WALLET.

AI summary:

The outcome is that the test flag should not be used in production, and a hardware wallet or multisig is recommended for a mainnet validator. The action items are to update the Celestia docs to clarify the keyring setup for mainnet validators and to test the setup on testnet first.