Add information about Singularity-in-Singularity

[aperloff]

Add a bonus lesson about using Singularity-in-Singularity. We are aware that most of the configuration changes will need to be done at the system administration level. However, it would still be useful for users to know:

1. what settings need to be active to make this possible (i.e. what they need to request from the sys admins)
2. how to test that "unprivileged user namespace" support is active (admin guide, example)

@kpedro88