Add grpcurl transaction submission example to cardano-rpc README

[carbolymer]

Context

The cardano-rpc README walks through read-only calls with grpcurl and buf, but it had no example of submitting a transaction, which is the first thing a user coming from cardano-submit-api looks for.
This PR adds a "Submit a transaction" subsection to the "Make your first call" section.
It shows how to turn a transaction signed with cardano-cli into a SubmitTx request and how to decode the returned reference into the usual hex transaction id.
It also notes that the transaction must be built for the node's current era, and that a rejected transaction comes back as a gRPC error carrying the ledger's reason.
The quickstart shell and the ad hoc tool line now also provide jq and xxd, which the example uses.

How to trust this PR

Documentation only, no code changes.
The submission was exercised against a throwaway three-node cardano-testnet cluster started with the gRPC HTTP listener.
The call returned a reference, the decoded reference matched the output of cardano-cli transaction txid byte for byte, the output appeared on chain, and submitting the same transaction again returned a gRPC error with the ledger's "All inputs are spent" message.
The README's pipeline differs from that run only in reading the request from stdin and in a portable base64 invocation.
To reproduce, start the cluster as the README describes, build and sign a transaction with one of the testnet's genesis UTxO keys, and run the commands from the new subsection.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Copilot review overview

🟢 Approval recommended

The documented request format and response decoding match the protobuf schema and server implementation.

Review effort: Balanced
Findings: None

What changed in this PR

Adds a practical grpcurl transaction-submission example to the cardano-rpc quickstart.

Changes:

• Documents transaction encoding, submission, and transaction-ID decoding.
• Adds jq and xxd to quickstart tooling.
• Adds the required documentation changelog fragment.

File	Description
cardano-rpc/​README.md	Adds the grpcurl submission walkthrough.
cardano-rpc/​quickstart/​shell.nix	Provides tools required by the example.
.changes/​20261007_cardano_rpc_readme_grpcurl_submit.yml	Records the documentation update.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.