Document Singularity as a topic/concept

[unito-bot]

Toil supports Singularity in its CWL and WDL runners. But we don’t really have any documentation that answers the question “How do I use Singularity with Toil?”, especially in the Slurm context.

We should explain Singularity and how to turn it on and off in the different front-ends.

┆Issue is synchronized with this Jira Story ┆Issue Number: TOIL-1437