docs: add extensions reactive upgrade node

doc/reactive-migration.md

@@ -0,0 +1,45 @@
+---
+uid: Extensions.Migration.Reactive
+---
+
+# Upgrading Extensions Reactive Version
+
+## Upgrading to Extensions Reactive V5
+
+Upgrading to Uno.Extension.Reactive V5 should not require any changes to your application code. However, there are new features and improvements that you may want to take advantage of.
+
+### MVUX Generated Code
+
+In V4, the MVUX analyzer generates a bindable proxy for each of the models in your app. For example, the bindable proxy for `MainModel` was named `BindableMainModel`.
+
+This behavior has changed in V5, and the MVUX analyzer will now generate a ViewModel class for each of your app's models. For example, the generated code for `MainModel` will be named `MainViewModel`.
+
+> [!IMPORTANT]
+> You don't have to do anything if you update but want to keep the old behavior. The generator will continue to generate the bindable proxies.
+
+To upgrade your application and use the latest generator, you must set an assembly level attribute, `BindableGenerationTool`, in the `GlobalUsing.cs`.
+
+```csharp
+// GlobalUsing.cs
+[assembly: Uno.Extensions.Reactive.Config.BindableGenerationTool(3)]
+```
+
+You will also need to update the references to the generated classes. These are usually referenced in the code-behind file when using XAML, for example, `MainPage.xaml.cs`, or in the Page UI definition when using C# for Markup, for example, `MainPage.cs`.
+
+```csharp
+// V4
+public MainPage()
+{
+    this.InitializeComponent();
+    DataContext = new BindableMainModel();
+}
+```
+
+```csharp
+// V5
+public MainPage()
+{
+    this.InitializeComponent();
+    DataContext = new MainViewModel();
+}
+```

doc/toc.yml

@@ -56,6 +56,8 @@
       items:
         - name: Overview
           href: Reference/Reactive/concept.md
+        - name: Upgrading Extensions Reactive
+          href: reactive-migration.md
         - name: General guidelines
           href: Reference/Reactive/general.md
         - name: Feed