Improve instructions and notebook for how to run on SLURM

[astronomerritt]

Because the existing ones are confusing.

Also, documentation page - example of just the demo command :)

[bernardinelli]

I was going through the issues and saw this. Would my distributing script (that @Gerenjie is using for his runs) help? It uses an unusual package (that I think we shouldn't have as a sorcha-wide dependency), but maybe for an example script it could be useful.