Custom splash screens for per-platform scenarios

The [splash screen doc](https://learn.microsoft.com/dotnet/maui/user-interface/images/splashscreen?view=net-maui-7.0&tabs=android) says that "For more advanced splash screen scenarios, per-platform approaches apply." Those per-platform approaches need doc'ing.

There have been reports of users being able to add per-platform splash screens on Android and iOS, although iOS easily produces storyboard errors. https://github.com/dotnet/maui/issues/12917 mentions how to do this on iOS.