docs: Add real staking transaction example for covenant-signer testing

[samricotta]

Currently, the documentation uses placeholder values for the test transaction:

curl -X POST http://127.0.0.1:9791/v1/sign-transactions -d '{
    "staking_tx_hex": "020000000001...",  # Placeholder
    # ... other fields
}'

We need to provide:

1. A real, complete staking transaction that operators can use for testing
2. The corresponding values for all related fields (slashing_tx_hex, scripts, etc.)
3. The expected signature output for this specific transaction