Proposal:WinUI 3 Template missing ContentDialog template

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### Title

WinUI 3 Template missing ContentDialog template

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WinUI 3 模板缺少 ContentDialog 模板

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As shown in the figure, in WinRT Xaml (UWP), the template created by ContentDialog is included, but it is not in WinUI 3. It is hoped that the missing ContentDialog template can be added as soon as possible

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如图所示，在 WinRT Xaml（UWP）中，包含了 ContentDialog 创建的模板，但 WinUI 3 却没有，希望能尽快添加缺失的 ContentDialog 模板

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WinRT Xaml（UWP）

WinUI 3

### Summary

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### Rationale

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### Scope

| Capability | Priority |
| :---------- | :------- |
| This proposal will allow developers to accomplish W | Must |
| This proposal will allow end users to accomplish X | Should |
| This proposal will allow developers to accomplish Y | Could |
| This proposal will allow end users to accomplish Z | Won't |
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### Important Notes

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### Open Questions

_No response_

Start by comparing the existing WinRT Xaml (UWP) ContentDialog template with the available WinUI 3 templates referenced in the issue. Confirm where template definitions are maintained and verify completion by making a ContentDialog template available for WinUI 3 users.

Written by the indexing model from the issue text.