Create the simplest sample that uses RNW in a Xaml islands

Xaml Islands are a complicated topic, and we get lots and lots of questions about them (and bugs that happen on them).

• There's a nice article that walks through adding RNW to a WPF app using Xaml Islands. But that's a slice in time, currently 2 years old, and not accessible as a sample that people can quickly build and copy.
• There's an existing repo of Xaml Islands Samples, but that's not going to include RNW.
• We have another issue on this repo tracking the need for a write up on this topic.
• Playground Win32 uses islands, but isn't documented and also is serving many more purposes than a "hello world w/ islands" app would.

I think what need is the simplest example app we can muster of a Xaml islands + RNW app. And we need it to live here on this repo alongside our other samples.

Start by examining the existing samples in this repository and the Playground Win32 implementation under packages/playground, then compare them with the linked Xaml Islands Samples and WPF article. Done means the repository contains the simplest buildable, copyable Xaml Islands app that uses RNW and is documented alongside the other samples.