Add Azure Authentication Docs

[richlander]

As we start documenting app-models with UI, we need to include topics like this for each one of them

https://developer.xamarin.com/guides/xamarin-forms/web-services/authentication/azure/

We can close this issue once we do it for the first one, which I assume we'll be for UWP. It's mostly just a reminder that we need to do this.

[mairaw]

Shouldn't this be covered on the Azure side of the house like https://docs.microsoft.com/en-us/azure/app-service-mobile/ @richlander?

[BillWagner]

This issue has been closed as part of the issue backlog grooming process outlined in #22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.