docs: Document platform-specific purchase handlers for React Native SDK

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Updates the Custom Purchase Config section to document the new platform-specific purchase handlers (makePurchaseIOS and makePurchaseAndroid) from PR #42. Includes Info callout explaining Android subscription parameters (basePlanId, offerId) and notes that the deprecated makePurchase handler is still supported for backward compatibility.

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Note

Updates React Native quickstart docs to use platform-specific purchase handlers with Android subscription params, noting deprecation of the old handler.

• Docs (React Native Quickstart)
  • Custom Purchase Config: Replace makePurchase with platform-specific makePurchaseIOS and makePurchaseAndroid in both Expo 52+ and Older Expo / Bare examples.
    • Android handler signature includes optional basePlanId and offerId.
  • Callouts:
    • Add Info on Android subscription parameters (basePlanId, offerId).
    • Add Note that deprecated makePurchase remains supported but recommend new handlers.

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