I just had the same issue – only I noticed that I was able to add a HTML-file. Once opened, simply switch to “Design” at the bottom of the screen.
You may want to change Build Action from ‘Content’ to ‘None’
As it is a hard-coded HTML document, it is also possible to use inline pictures (e.g. a diagram)
Also for my purpose (programming guide, architecture description. database use examples) I opted to create a separate project (_Documentation) as a Windows Forms, as this will allow me (or a new programmer) to have a running example.