MarkdownTextBlock: add alternative to Text property to pass in a MarkdownDocument to the control

[jcoc611-microsoft]

Currently, the only way to set the content of a MarkdownTextBlock is via the Text property, which internally uses Markdown.Parse to convert to a MarkdownDocument. It would be nice to also allow users to pass in an existing MarkdownDocument as an alternative.
 
Example:

<controls:MarkdownTextBlock
  MarkdownDocument="{x:Bind Doc, Mode=OneTime}" />

Benefits:

• Users can insert arbitrary logic after Markdown.Parse(...), such as mutating the MarkdownDocument tree with decorations
• Users can provide a custom Markdown pipeline
• Users can preserve the MarkdownDocument for other operations, such as copying the content to HTML with Markdown.ToHtml(...)