docs: clarify custom axis labels

[huytdps13400]

Summary

• clarify that AxisLabel is app-owned rather than exported by the package
• explain the zero-argument label callback contract and the reserved axis rows
• replace the undefined component example with a complete React Native Text implementation that derives min/max values

Fixes #85.

Verification

Using the repository Node.js version (22.20.0) with fail-fast shell execution:

• 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-85-web-dist
• git diff --check

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

[chrispader]

LGTM, thanks!