Faillure rendering sample docs.

[Rosuavio]

Describe the bug

A clear and concise description of what the bug is.
Some doc pages fail to render in the sample app due to not being able to handle certain markdown.
When parsing special quotes with special style tags like [!NOTE] or [!TIP], the sample app does not handle there not being one of those style tags at the beginning of the quote and the example code would crash the rendered.

In the SampleApp there are docs that we render that have the markdown syntax

> Hello world

And should be rendered as

Hello world

But instead the doc fails to render and none of the doc page is shown.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? (Either in a sample as-is or with new XAML pasted in the editor.) If so, please provide custom XAML or steps to reproduce. If not, let us know why it can't be reproduced (e.g. more complex setup, environment, dependencies, etc...)

Steps to reproduce the behavior:

1. Open SampleApp
2. Open login button sample
3. See no docs.

Expected behavior

A clear and concise description of what you expected to happen.
Login button docs and other docs with the md syntax > work.

Screenshots

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Environment

NuGet Package(s):

Package Version(s):

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [ ] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio
- [ ] 2017 (version: )
- [ ] 2019 (version: )
- [ ] 2019 Preview (version: )

Additional context

Add any other context about the problem here.