You get links, if you need more than that you're probably overcomplicating your documentation. Transclusion might sound nice, but now your docs have depedencies and they become harder to send, harder to host, and harder to edit (is this edit in this transcluded file going to work for every other file that includes it?). Then someone goes "I use it for templating" and now you have a CMS. It's a slippery slop that markdown sidesteps by going "no. You get links. If you need more than links, you're probably overcomplicating your documentation".