Add multi-head template for migration

[DianaSoltani]

Todo before merge:

• waiting on merge: https://github.com/dotnet/maui/pull/20365
  • Retrieve Net 8 version that this will be released on to add as a note to the Multi Template section
• add more information on app entry points?

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Internal previews

📄 File	🔗 Preview link
docs/migration/multi-project-to-multi-project.md	Manually upgrade a Xamarin.Forms app to a multi-project .NET MAUI app