[MarkdownTextBlock] Improve sample page in Labs gallery

### Describe the bug

There are a couple of things we could do to improve the sample page.

- [ ] Fix the markdown sample text we use in the built-in example. There are things in it that doesn't work or isn't implemented.
- [ ] Add a new section for streaming rendering

### Steps to reproduce

```text
Observe the sample page in Labs gallery app
```

### Expected behavior

The sample page should not contain markdown example that doesn't work.
There should be a streaming rendering example

### Screenshots

_No response_

### Code Platform

- [x] 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

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