move documentation dependencies to Build-Depends-Indep
there are more unsatisfiable cross Build-Depends left. I've found another opportunity for reduction though: Skipping the documentation build during arch-only. Once doing so, we can demode sphinx and numpydoc to Build-Depends-Indep. Doing so requires changing debian/rules though.
The sphinx gallery triggers run of example code which relies from line one on Internet access to operate properly, which is a policy violation for compiling binary packages, and a problem in general when the remote resource goes unavailable.
