logbook cannot be cross built from source due to unsatisfiable cross
Build-Depends relating to sphinxdoc. I note that its documentation is
neatly separated into an arch:all -doc package, so I attempted moving
those Build-Depends to Build-Depends-Indep making them irrelevant to
cross building. That readily succeeded and I verified (using
reproducible builds) that the artifacts of a thus modified arch-only
build exactly reproduce the shared artifacts with a full build on amd64.
Please find a patch attached.

Helmut