Implementing and testing in-app purchases with StoreKit2 and StoreHelper in Xcode 13 - 15 with SwiftUI, Swift 5.7 - 5.9, iOS 15 - 17 and macOS 12 - 14.

• StoreHelper Documentation Landing Page - StoreHelper documentation landing page
• StoreHelper Quick Start - StoreHelper quick-start guide
• StoreHelper In-Depth Guide - StoreHelper and StoreKit2 in-depth guide
• StoreHelper Demo with UIKit - experimental demo showing StoreHelper in a UIKit project

• Updated demo project minimum iOS target to iOS 17.0 and macOS to 14.0
• Added views to show how to integrate StoreKit's StoreView, ProductView, and SubscriptionStoreView
• Replaced deprecated NavigationView with NavigationStack in MainView