adding explanations for running jobs on O2

[mistrm82]

Add some text before and after the sbatch exercise (within the markdown) to explain:

• two commands to submit jobs on O2: srun and sbatch (at the beginning). Highlight that each requires you to request resources. Explain the differences (when to use each)
• (At the end?) Describe that you can also run sbatch the same way you do as srun (single line command)

@jihe-liu I think this issue covers what is described in https://github.com/hbctraining/Intro-to-shell-flipped/issues/6 ? If so, let's consolidate and close the other issue