Markdown files not in samples project root don't get loaded in dev app

### Describe the bug

For https://github.com/CommunityToolkit/Windows/pull/136 and https://github.com/CommunityToolkit/Windows/pull/141 - I wanted to place the Markdown document in the folder along with the samples.

However, when I ran the dev app, I noticed the sample page within the folder wasn't listed... i.e. missing

We should support having the Markdown/Samples in any folder structure within the Sample project.

https://github.com/CommunityToolkit/Tooling-Windows-Submodule/blob/38728ba616661853b3bbcec9269ab1e362daa72a/ToolkitComponent.SampleProject.props#L40

Seems to indicate that we should be adding additional files from all subdirectories in the project, so maybe this is a generator issue?

### Steps to reproduce

```text
1. Open a component in VS (stand-alone w/ OpenSolution.bat)
2. Open the samples project (You can run app to verify all docs show up on right after samples)
3. Add a folder
4. Move a Markdown file into (probably outside of VS, i.e. no changes to project file)
5. Run App

Notice that the document in the folder isn't listed anymore as a tab...
```

### Expected behavior

Documentation should appear as a tab as it was before when in root folder.

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [X] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

Yes, but only if others can assist.

40行目付近の ToolkitComponent.SampleProject.props から始め、samples の dev app がプロジェクトルート外の Markdown ファイルをどのように検出しているかを追跡します。プロジェクトファイルを変更せずに Markdown ファイルをサブフォルダーへ移動して再現します。完了の条件は、ルートレベルのドキュメントと同じように、ネストされたドキュメントがタブとして表示されることです。

索引モデルが issue の本文から書いたものです。