docs: add a reproducible real-contract Soroban testnet walkthrough

[naninu123]

Problem

The CLI documentation explains individual commands well, but a new user still needs a single reproducible workflow against a real public Soroban testnet contract. A maintained walkthrough would demonstrate that sdkt works beyond repository fixtures and would provide a useful adoption reference.

Scope

Create a guide that uses a stable public testnet contract and covers:

1. creating or selecting a testnet network profile;
2. sdkt inspect and sdkt health;
3. event retrieval;
4. storage inspection;
5. structured JSON output suitable for CI;
6. expected output fields and common RPC errors.

Do not commit secrets. Do not depend on a temporary account controlled by one contributor.

Acceptance criteria

• Commands run from a clean checkout with the documented prerequisites.
• The contract ID and network passphrase are explicit.
• Output examples match the current CLI schema.
• A smoke script or network-gated test validates the walkthrough.
• CI remains green when network tests are skipped.