Slurm docs overhaul

[LinusSch]

I'm about to put our Slurm doc pages into the navigation menu and make us a structure for this. Currently, I think we should have four pages:

• Slurm, the job scheduler: introduction to and overview of Slurm
• Slurm on Pelle: recommended usage for Pelle specifically
• Slurm on Bianca: recommended usage for Bianca specifically
• Slurm troubleshooting: how to handle the most common errors, common Uppmax-specific errors, how to ask us for help

I was considering a fifth page but I changed my mind: I think we should rather be linking heavily to the course material, for generally recommended usage and tips and tricks, script examples, deeper explanations. Or is that a bad idea? Should we aim for moving that info into the upcoming Naiss-wide docs and link to there?

For now I will put the first three pages in the navigation and do what I can for the intro/overview page and the Pelle page. Troubleshooting and details and whatever other pages I can find will have to wait.

[LinusSch]

For now I have left the troubleshooting page out of the navigation, as I feel it needs to into a more efficient FAQ before it makes sense to surface it like that. (In our current scheme, where some pages are surfaced in navigation and some are not, which I'm leaning strongly towards doing away with.)