feat: add sample in web app.

demo/Semi.Avalonia.Demo/Pages/NotificationDemo.axaml.cs

@@ -0,0 +1,34 @@
+using System;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Notifications;
+using Avalonia.Controls.Presenters;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+using Avalonia.VisualTree;
+using Semi.Avalonia.Demo.Views;
+
+namespace Semi.Avalonia.Demo.Pages;
+
+public partial class NotificationDemo : UserControl
+{
+    private MainWindow? _window;
+    public NotificationDemo()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        _window = VisualRoot as MainWindow;
+    }
+
+    private void InfoButton_OnClick(object? sender, RoutedEventArgs e)
+    {
+        if (sender is Button b && b.Content is string s && Enum.TryParse<NotificationType>(s, out NotificationType t))
+        {
+            _window?.Notify(t);
+        }
+    }
+}