step-40: update IO section

step-40: update IO section

Update the documentation on how we generate output using MPI I/O (we
originally wrote one vtu per process).
Remove the limit of 32 MPI ranks, because it is confusing according to
my students. I think this was put in place to avoid generating 1000+
output files, which is no longer the case (we use 8 here). The output is
a few MB in total, so I don't see a reason to limit things.