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

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:

- [x] #2476
- [x] #2470
- [x] #2402
- [x] #2403
- [x] #2346
- [x] #1300