Add documentation for macOS add-to-app `flutter build macos-framework`

[jmagman]

What information is missing?

flutter build macos-framework is now available in the master channel and should be documented.

How would you like us to fix this problem?

Add macOS versions of https://docs.flutter.dev/development/add-to-app/ios/project-setup and https://docs.flutter.dev/development/add-to-app/ios/add-flutter-screen. This doesn't work from from a module (only normal flutter created app), and only option B and option C are available.