Add YAML Frontmatter extraction support

[sean-r-williams]

Related to powershell/powershell#16857.

This is already a native feature of Markdig, so it should be as easy as calling .UseYamlFrontMatter() on MarkdownPipelineBuilder before .Build().

Given that these are essentially document properties, a property added to [Microsoft.PowerShell.MarkdownRender.MarkdownInfo] instances that extracts YAML frontmatter (if present) and deserializes to a CLR object would likely be the most sensible to use.