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(...)

[jcoc611-microsoft]

Thinking about it more, I think it is actually better to expose the control's MarkdownDocument rather than allowing users to pass one in.

Users can insert arbitrary logic after Markdown.Parse(...)

This should only be done via Markdig extensions. We will provide different extensibility mechanisms which also allow for implementing custom renderers for the outputs of the extensions.

Users can provide a custom Markdown pipeline

This is not really desired in general, other than extensions.

Users can preserve the MarkdownDocument for other operations

The control owns the MarkdownDocument and users can just access it directly

TBD: add an event for when parsing is done (before this, accessing MarkdownDocument is not valid, and it would be null)