☂ [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:

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