Two Islands Example

Could be useful to have an example of showing how to use Two different Islands.

For instance maybe there's a `TabbedCommandBar` or `TabView` as the top-level navigation, but then a `DataGrid` is used as the main page's content.

首先查看现有示例结构，了解 Windows Community Toolkit 控件在 C++/WinRT UWP 应用程序中的演示方式。添加一个 Two Islands 示例，使用 TabbedCommandBar 或 TabView 实现顶层导航，并使用 DataGrid 显示主页内容；完成标准是示例清楚地展示这两个控件协同工作。

由索引模型根据 Issue 内容生成。