Setup guide for BN missing core gRPC

The setup guide for bridge nodes doesn't first ensure that gRPC is enabled on the core node. That dependency just appears out of nowhere. The default initialization of `grpc_laddr` in `config.toml` for core will be `""` instead of `"tcp://127.0.0.1:9090"` for example.

https://github.com/celestiaorg/docs/blob/c4be5c1189029574ed4c60144309a15c09159aab/how-to-guides/bridge-node.md#L74