☂ [add-to-app] Simplify add-to-app samples

[ericwindmill]

There should only be one add-to-app Quickstart that demos the recommended way to add Flutter to each platform (iOS, android app, web, macOS, windows,)

We don't need samples to demonstrate each possible way to embed a Flutter app, and we shouldn't demonstrate anything that requires a lot of explanation to understand. Complex information should be written about in documentation.

The following issues should be closed as part of this work:

• [add-to-app] Accessibility not working properly with multiple flutter fragments  #2476
• [add-to-app] Build failed with an exception. #2470
• [add-to-app] Wish for the add_to_app samples using SwiftUI to match the new updated docs #2402
• Wish for a sample of a non-fullscreen Flutter view displayed from a full SwiftUI app #2403
• Update dependencies for add to app examples #2346
• [add-to-app/prebuilt_module] Add macos-framework example #1300