Add more info about DTNs

[tom-papatheodore]

We might want to add some minimal information about the DTNs (e.g., CPUs, scheduler) to https://docs.olcf.ornl.gov/systems/dtn_user_guide.html. I was able to find some information on the old Rhea page (https://www.olcf.ornl.gov/for-users/system-user-guides/rhea/running-jobs/) - such as the batch scheduler.

[jack-morrison]

@tom-papatheodore is this a captured by #128?