docs: add a step to enable gRPC for the bridge node setup

[tksohishi]

Problem

When connecting the bridge node to the consensus node running on localhost, I believe we should explicitly enable the gRPC configuration in config/app.toml. It's relevant to the step described in https://docs.celestia.org/nodes/validator-node#run-the-bridge-node and closely related to https://docs.celestia.org/nodes/full-consensus-node#expose-rpc

Proposed changes

• Add a little more explanations to "TIP" in Initialize the bridge node
• Tweak a little on Optional: Configuring an RPC endpoint section to mention the configuration above

If that makes sense, I'd create a PR to modify the documents!

[jcstein]

makes sense! thank you @tksohishi