NativeAOT: Document support for NativeAOT runtime with Xamarin

[ivanpovazan]

We should document how NativeAOT runtime is supported with Xamarin as an experimental feature for the .NET8 GA release.
The documentation should include the description of supported features, build integration (like: CustomNativeMain), added components (like: https://github.com/xamarin/xamarin-macios/blob/main/docs/managed-static-registrar.md), compatibility issues, etc.

PS I have created this tracking issue as a placeholder. Feel free to adapt it and/or add more context.