Create public OSS sample app(s) using React Native Fabric

[jonthysell]

Publicly available sample applications which demonstrate how to set up and use a React Native Windows Fabric Island in a Win32 host window. This also includes all requisite companion documentation. Without this, we won’t be able to easily determine an MVP nor test and verify our proposed Island API surface with end developers.

Tasks

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1. Publish a sample-app-fabric to the react native windows repo #13389
   Area: Fabric Area: Islands Area: Samples New Architecture +4
   
2. Enable ado on the sample-app-fabric #13385
   Area: Fabric Area: Islands Area: Samples New Architecture +4
   
3. Introduce ReactNativeAppBuilder and ReactInstanceSettingsBuilder #13387
   Area: Fabric Area: Islands Area: Samples New Architecture +4
   
4. Load failed error for Shared.vcxitems in sample-app-fabric #13384
   Area: Fabric Area: Islands Area: Samples Invalid Triage New Architecture +5
5. Update sample-app-fabric to use ReactNativeAppBuilder #13388
   Area: Fabric Area: Islands Area: Samples New Architecture enhancement +5
   
6. Showcase a js-app scenario to be hosted through the ReactNativeIsland
7. Update Public documentation for creating Win32 Fabric Application
8. Create Public documentation to set up and use ReactNative Island
9. Fabric template app source is hard to navigate / customize #12180
   0 of 1
   Area: App Template Area: Fabric New Architecture enhancement +4
   

[chrisglein]

This is for the sample apps in the RNW repo itself. Distinct from the external samples in react-native-windows-samples, which would be a different step (if that warrants an specific sample more than what's in repo, since the default app template is expected to switch to using ReactNativeIsland.