cardano-rpc: Add quickstart documentation and examples

[carbolymer]

Context

Adds a Quickstart section to cardano-rpc/README.md and per-language example clients under cardano-rpc/quickstart/.
The goal is to make cardano-rpc approachable for external developers, so someone new to the project can get from a fresh checkout to a working call in a few commands, without first reading the proto files or the Haskell server code.

The quickstart covers:

• Prerequisites (Nix with flakes, a checkout of this repository for the vendored proto files).
• Starting a local single-node cluster with cardano-testnet --enable-grpc.
• Bridging the cluster's Unix-socket gRPC endpoint to TCP with socat, for tools that expect TCP.
• Making first calls with buf and grpcurl.
• Cleaning up the cluster afterwards.
• A configuration reference for the three transport modes (Unix socket, HTTP/2 cleartext, HTTP/2 with TLS).

Two language examples are included:

• Rust, using the utxorpc-spec crate's v1beta bindings.
• TypeScript, with a hand-rolled v1beta MeshJS provider. The published JS u5c SDKs are still pinned to v1alpha (tracked upstream in chore: initialize v1 version utxorpc/spec#209), so @meshsdk/provider's off-the-shelf U5CProvider does not work against cardano-rpc yet.

How to trust this PR

All commands and examples in this PR were run against a live local cluster started the same way the Quickstart describes.

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/

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