Apptainer docs improvements

[jen-reeve]

Improve Apptainer page

Currently mostly looks like a bunch of warnings, doesn't really encourage use, honestly scares me

[jen-reeve]

Examples of Apptainer docs from other HPC systems:

• https://researchcomputing.princeton.edu/support/knowledge-base/apptainer
• https://www.carc.usc.edu/user-guides/hpc-systems/software/apptainer
• https://hpc.nih.gov/apps/apptainer.html

[WesHarrell]

I did a cleanup merged in 1060, but could go add and expand content