Add article on reconciling RNW's relationship with Win32

[chrisglein]

We start by focusing RNW on UWP. I feel like that story is pretty clear.
However there are 2 options for working with Win32:

• Using XAML Islands (see this article at the "Embedding the React Native component inside a Win32 app" section)
• Using the React Native for Win32 solution that emits Win32 controls instead of XAML controls (which is also part of the react-native-windows repo)

This difference can be confusing, we should get out in front of that with documentation.

[chrisglein]

I think we cover this well enough here: https://microsoft.github.io/react-native-windows/docs/native-code#win32-desktop-apps-vs-rnw-apps
Further evolution should be covered by WinAppSDK documentation.