There are some forum softwares that allow people to create custom pages using BBCode (in this case it would be markdown). Here is how I would see it:
At the bottom of the Content tab in the admin panel would be a button that said “new”. When you click it, you can enter a title, slug (url), and content. Pretty simple I guess.
There is a branch created by another user so it might now be that hard to implement. https://github.com/potthast/discourse/tree/custom-static-pages
Maybe before 1.0.0?