removing -B parameter from the first section. Adding a method to look at the content of the singularity image

Update singularity_container.ipynb

See merge request rc-training-sessions/singularity_containers!1

Correct some typos
Reduce wording in first example around neurodebian container description.
Try to improve some of the descriptions around running singularity as a command in a batch script.

* Added another example to pull from Nvidia GPU Cloud.
* Also added methods of creating your own recipe files.
* Added a separate section for acknowledgment/important links.