Improve multi-platform targeting documentation

[santo998]

Based on this StackOverflow question: https://stackoverflow.com/questions/77534919/how-to-share-code-between-ios-and-mac-on-net-maui-no-conditional-compilation

I think you should improve this documentation: https://github.com/dotnet/docs-maui/blob/main/docs/platform-integration/configure-multi-targeting.md

Clarifying that when using "folder-based multi-targeting" we have to use our own root folder, and not the standard "Platforms" folder.

Also, I think you should add some folder structures samples, like the accepted answer of that StackOverflow link has.