Fix MPI Documentation

[b-butler]

Issue Description

MPI documentation is currently wrong in recipes.rst. It seems to predate nranks and definitely predates groups which enforced the submit -> run -> {exec,run in process} pipline. This means the documentation is out of date and actually wrong in some places.

Proposed change (optional)

Redo the documentation to use nranks and remove template customization for MPI operations.