Docs - Add Migration Guide for NotifyTaskCompletion

As part of https://github.com/windows-toolkit/WindowsCommunityToolkit/pull/3435, we're removing the `NotifyTaskCompletion` helper from the Toolkit. We should have a section in the docs for [AsyncRelayCommand](https://github.com/windows-toolkit/MVVM-Samples/blob/master/docs/mvvm/AsyncRelayCommand.md) on how to migrate code from that pattern to this one.

Or maybe it's in [ObservableObject](https://github.com/windows-toolkit/MVVM-Samples/blob/master/docs/mvvm/ObservableObject.md#handling-taskt-properties). Both places may want to call something out, maybe they both link to a separate page?

Just want to make sure we show folks that used `NotifyTaskCompletion` in C#/XAML how to convert that over with the full C#/XAML pattern in old and new style shown.