Create extensions for DoxFX-flavoured markdown

[Cloud-Awesome]

1. Yaml Header - related to Add DocFx metadata element #1
2. Warnings, tips, info boxes
3. Tabbed content

Also consider (but maybe later)

1. Include code from source
2. Section (Useful for adding custom CSS classes or JS events?)
   • e.g. see this MS example which is rendered like this

c.f. https://dotnet.github.io/docfx/spec/docfx_flavored_markdown.html?tabs=tabid-1%2Ctabid-a

[Cloud-Awesome]

As these are DocFX specific, are (intentional) variations away from GFM(?) they should be included in an explicit separate namespace.

e.g. something like

var warningBox = new DocFx.MdInfoBox("This is the content of the box", MdInfoBoxType.Warning)

which produces:

> [!WARNING]
> This is the content of the box
>

[Cloud-Awesome]

Keep in mind conversion to PDF should always be supported. Each component needs to be supported for web and PDF (preferably).

e.g. the tabbed content wouldn't translate very well into a PDF? ;)