docs(sdk): quickstart example — query a user's history in 5 lines

[Omoboi-dev]

Context
A short, copy-pasteable example is the best advertisement for the SDK.

Task
Write a quickstart that shows installing the SDK and querying a user's history
in a few lines, with expected output.

Pointers

• Add to docs/ and link from the README + packages/sdk.
• Use the real StardexClient API.

Acceptance criteria

• A runnable, minimal example with expected output
• Linked from the README
• Stays accurate to the current SDK API

Notes
🟢 Good first issue.