docs: add a worked end-to-end example from metrics to on-chain query

[Hollujay]

Summary

The README covers the CLI and the architecture separately. There is no single
walkthrough that takes a reader from a running node all the way to querying
that provider's history back off-chain.

Acceptance Criteria

• Add a walkthrough (README section or docs/ page) covering: deploy the
  contract to testnet, register a provider, parse metrics with the CLI,
  submit the attestation, then read the history back
• Use real commands with real output, not placeholders
• State clearly which steps cost fees and which network is assumed
• Someone unfamiliar with the repo should be able to follow it start to
  finish without guessing

Tech Stack

Markdown, Stellar CLI