Add support for creating additional kinds of documents

[Cronan]

E.G. https://www.mkdocs.org

[govert]

Are you using MkDocs? I've had a quick look at options for general Excel-DNA documentation, including DocFx, but that seems a bit immature still.

[Cronan]

Yes, we're using it at work, it's nice because it ties directly to an internal bitbucket repo