docs: disable doctests by default, allow enabling

[fingolfin]

I am not at a computer right now, else I'd do this right away, but I don't want to forget about this (again): we wanted to disable doctests when building the docs by default, and add a keyword argument to Oscar.build_doc to allow enabling them again. Perhaps also add other KW args to it, e.g. for controlling the strict argument of the doit function. (And perhaps some of the args of doit should be turned into KW args, too?)