docs: restructure documentation to align with Open IAP specification

- Add Open IAP conformance statement across all documentation
- Redirect iOS/Android setup guides to openiap.dev for unified setup instructions
- Redirect lifecycle documentation to Open IAP specification
- Add sponsors page with redirect to unified sponsorship program
- Update README with left-aligned layout and OpenIAP links
- Remove duplicate setup content to avoid confusion between spec and implementation
- Maintain implementation-specific code examples and integration guides

This restructuring separates specification documentation (managed on openiap.dev)
from implementation documentation (flutter_inapp_purchase specific), providing clearer
guidance for developers while avoiding content duplication.

Author: hyochan

README.md

@@ -1,25 +1,14 @@
 # flutter_inapp_purchase
 
-<p align="center">
-  <img src="https://flutter-inapp-purchase.hyo.dev/img/logo.png" width="200" alt="flutter_inapp_purchase logo" />
-</p>
+<img src="https://flutter-inapp-purchase.hyo.dev/img/logo.png" width="200" alt="flutter_inapp_purchase logo" />
 
-<p align="center">
-  <a href="https://pub.dartlang.org/packages/flutter_inapp_purchase"><img src="https://img.shields.io/pub/v/flutter_inapp_purchase.svg?style=flat-square" alt="Pub Version" /></a>
-  <a href="https://github.com/hyochan/flutter_inapp_purchase/actions/workflows/ci.yml"><img src="https://github.com/hyochan/flutter_inapp_purchase/actions/workflows/ci.yml/badge.svg" alt="Flutter CI" /></a>
-  <a href="https://codecov.io/gh/hyochan/flutter_inapp_purchase"><img src="https://codecov.io/gh/hyochan/flutter_inapp_purchase/branch/main/graph/badge.svg?token=WXBlKvRB2G" alt="Coverage Status" /></a>
-  <img src="https://img.shields.io/badge/license-MIT-blue.svg" alt="License" />
-</p>
+[![Pub Version](https://img.shields.io/pub/v/flutter_inapp_purchase.svg?style=flat-square)](https://pub.dartlang.org/packages/flutter_inapp_purchase) [![Flutter CI](https://github.com/hyochan/flutter_inapp_purchase/actions/workflows/ci.yml/badge.svg)](https://github.com/hyochan/flutter_inapp_purchase/actions/workflows/ci.yml) [![Coverage Status](https://codecov.io/gh/hyochan/flutter_inapp_purchase/branch/main/graph/badge.svg?token=WXBlKvRB2G)](https://codecov.io/gh/hyochan/flutter_inapp_purchase) ![License](https://img.shields.io/badge/license-MIT-blue.svg)
 
-<p align="center">
-  A comprehensive Flutter plugin for implementing in-app purchases on iOS and Android platforms.
-</p>
+A comprehensive Flutter plugin for implementing in-app purchases that conforms to the [Open IAP specification](https://openiap.dev)
 
 ## 📚 Documentation
 
-For comprehensive documentation, installation guides, API reference, and examples, visit:
-
-**🌐 [flutter-inapp-purchase.hyo.dev](https://flutter-inapp-purchase.hyo.dev)**
+**[📖 Visit our comprehensive documentation site →](https://flutter-inapp-purchase.hyo.dev)**
 
 ## 📦 Installation
 
@@ -49,6 +38,10 @@ await FlutterInappPurchase.instance.requestPurchase(
 );
 ```
 
+## Sponsors
+
+💼 **[View Our Sponsors](https://openiap.dev/sponsors)**
+
 ## 📄 License
 
 MIT License - see [LICENSE](LICENSE) file for details.