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.

Commencez par ToolkitComponent.SampleProject.props vers la ligne 40 et suivez la manière dont l’application de développement des samples découvre les fichiers Markdown en dehors de la racine du projet. Reproduisez le problème en déplaçant un fichier Markdown dans un sous-dossier sans modifier le fichier du projet ; le travail est terminé lorsque les documents imbriqués apparaissent comme des onglets, tout comme les documents situés à la racine.

Rédigé par le modèle d'indexation à partir du texte de l'issue.