Review Singularity installation document

With the release of Singularity 3 in the OSG 3.4 release repo, Singularity's security posture has changed for the better (SOFTWARE-3688, SOFTWARE-3708). Namely, there is no longer a separate singularity-runtime package intended just for the worker nodes. We should review the Singularity installation document to make sure that our security discussions reflect 3.2. For example, there is a link to a privilege escalation section in the 2.6 manual that no longer appears to exist in the 3.2 documentation.