chore: add avalonia CheckBox demo.

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Styles/CheckBoxDemoCtl.axaml

@@ -0,0 +1,26 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="HandyControlDemo.UserControl.CheckBoxDemoCtl">
+    <StackPanel Margin="32"
+                HorizontalAlignment="Center"
+                VerticalAlignment="Center">
+        <CheckBox Content="CheckBox"
+                  IsChecked="True" />
+        <CheckBox Margin="0,16,0,0"
+                  Content="CheckBox"
+                  IsChecked="True"
+                  IsEnabled="False" />
+        <CheckBox Margin="0,16,0,0"
+                  Content="CheckBox" />
+        <CheckBox Margin="0,16,0,0"
+                  Content="CheckBox"
+                  IsEnabled="False" />
+        <CheckBox Margin="0,16,0,0"
+                  Content="CheckBox"
+                  IsChecked="{x:Null}" />
+        <CheckBox Margin="0,16,0,0"
+                  Content="CheckBox"
+                  IsChecked="{x:Null}"
+                  IsEnabled="False" />
+    </StackPanel>
+</UserControl>

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Styles/CheckBoxDemoCtl.axaml.cs

@@ -0,0 +1,9 @@
+namespace HandyControlDemo.UserControl;
+
+public partial class CheckBoxDemoCtl : Avalonia.Controls.UserControl
+{
+    public CheckBoxDemoCtl()
+    {
+        InitializeComponent();
+    }
+}