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

From [Slack message](https://celestia-team.slack.com/archives/C08GGQFHM1S/p1743686792229149?thread_ts=1743439910.224859&cid=C08GGQFHM1S):
> 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.
> 1. 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.