Upgrade MarkdownTextBlock (#44793)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

This PR:

- upgrades `CommunityToolkit.WinUI.Labs.MarkdownTextBlock` to version
`0.1.260116-build.2514`. This update includes a bunch of improvements
for markdown rendering in its default config, and fixes a couple of bug
with regards to rendering large images getting clipped when resizing the
window.
- replaces an incorrect image in the `Command Palette Sample Page`
extension for the markdown + images sample.

Before vs after:

<img width="910" height="234" alt="image"
src="https://github.com/user-attachments/assets/b3dad76c-a89e-4b47-90f8-d3c64f00615f"
/>


<img width="1245" height="827" alt="image"
src="https://github.com/user-attachments/assets/00037fb5-453e-4d85-83c9-92c265b9f968"
/>



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## PR Checklist

- [ ] Closes: #xxx
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/ext/SamplePagesExtension/Pages/SampleMarkdownImagesPage.cs

@@ -69,14 +69,14 @@ internal sealed partial class SampleMarkdownImagesPage : ContentPage
 
             ### Web URL
             ```xml
-            ![painting](https://i.imgur.com/93XJSNh.png)
+            ![painting](https://raw.githubusercontent.com/microsoft/PowerToys/refs/heads/main/doc/images/overview/Original/AdvancedPaste.png)
             ```
-            ![painting](https://i.imgur.com/93XJSNh.png)
+            ![painting](https://raw.githubusercontent.com/microsoft/PowerToys/refs/heads/main/doc/images/overview/Original/AdvancedPaste.png)
             
             ```xml
-            ![painting](https://i.imgur.com/93XJSNh.png?--x-cmdpal-fit=fit)
+            ![painting](https://raw.githubusercontent.com/microsoft/PowerToys/refs/heads/main/doc/images/overview/Original/AdvancedPaste.png?--x-cmdpal-fit=fit)
             ```
-            ![painting](https://i.imgur.com/93XJSNh.png?--x-cmdpal-fit=fit)
+            ![painting](https://raw.githubusercontent.com/microsoft/PowerToys/refs/heads/main/doc/images/overview/Original/AdvancedPaste.png?--x-cmdpal-fit=fit)
 
             ### File URL (PNG):
             ```xml

src/modules/cmdpal/ext/SamplePagesExtension/Pages/SampleMarkdownPage.cs

@@ -81,7 +81,7 @@ Result:
 
 Result:
 
-![painting](https://i.imgur.com/93XJSNh.png)
+![painting](https://raw.githubusercontent.com/microsoft/PowerToys/refs/heads/main/doc/images/overview/Original/AdvancedPaste.png)
 
 ### Links