docs: make usage example self-contained

[huytdps13400]

Summary

• replace the undefined usePriceHistory() call with a complete GraphPoint[] example
• import the public GraphPoint type so the supported data shape is visible immediately
• clarify that consumers provide their own data and that the x-axis is date-based

Fixes #109.

Verification

Using the repository Node.js version (22.20.0):

• yarn prettier --check README.md
• yarn test --runInBand — 2 suites, 3 tests passed
• yarn typecheck
• yarn lint — 0 errors; one pre-existing no-shadow warning in src/AnimatedLineGraph.tsx
• yarn prepare
• CI=1 yarn workspace react-native-graph-example expo export --platform web --output-dir /tmp/graph-109-web-dist
• git diff --check

This is documentation-only; no runtime or public API behavior changes.

[chrispader]

LGTM, thanks!