feat: add sample in web app.

demo/Semi.Avalonia.Demo/Views/MainWindow.axaml.cs

@@ -0,0 +1,23 @@
+using Avalonia.Controls;
+using Avalonia.Controls.Notifications;
+
+namespace Semi.Avalonia.Demo.Views;
+
+public partial class MainWindow : Window
+{
+    private readonly WindowNotificationManager _manager;
+    public MainWindow()
+    {
+        InitializeComponent();
+        _manager = new WindowNotificationManager(this)
+        {
+            Position = NotificationPosition.TopLeft,
+            MaxItems = 3
+        };
+    }
+    
+    internal void Notify(NotificationType t)
+    {
+        _manager.Show(new Notification(t.ToString(), "This is a notification message", t));
+    }
+}