The Marketplace team is working on bug 889536, which will update their API documentation to use our Sphinx theme. Once that is done, we will need to push a change to our configs to actually host their docs on MDN.

FWIW, in discussions we've had since this bug was first opened, I think we've backed off from trying to get 3rd party documentation hosted from MDN URLs. Too many potential security issues, and the solutions we've come up with so far are probably too hacky to implement. So, having had the Marketplace API docs adopt an MDN theme is good enough, and having them available at this URL is fine: https://firefox-marketplace-api.readthedocs.org/en/latest/