Move and cleanup the cross-submission stuff from the Rhea users guide

[isubsmith]

Right now, the Rhea Users Guide is the only place where Cross-submission is discussed.

I think it needs moved to a higher level and then referenced in the summit guide and the rhea guide (similar to "Connecting" #55)

(It also needs slurm and lsf updates, and verified that it all still works)

[grahamlopez]

note: the cross-submission section was removed completely in #94, with the intention that it just needs to be re-written anyway.

[verolero86]

For the new cross-submission section, this works from Summit:

sbatch --cluster dtn some_slurm_script.sh
sbatch --cluster rhea some_slurm_script.sh