feat: add demo.

demo/Ursa.Demo/Pages/BreadcrumbDemo.axaml

@@ -0,0 +1,15 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:u="https://irihi.tech/ursa"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="Ursa.Demo.Pages.BreadcrumbDemo">
+    <StackPanel>
+        <u:Breadcrumb>
+            <TextBlock Text="Hello"></TextBlock>
+            <u:BreadcrumbItem Content="World" Icon="?"></u:BreadcrumbItem>
+        </u:Breadcrumb>
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Pages/BreadcrumbDemo.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class BreadcrumbDemo : UserControl
+{
+    public BreadcrumbDemo()
+    {
+        InitializeComponent();
+    }
+}