Add reference section for metrics collected by the Slurm prometheus exporter

[NucciTheBoss]

The Slurm prometheus exporter collects a lot of useful information that is fed into COS, but you might not know exactly what it means at face value. We should have a page in the reference section of our documentation that documents all the metrics that are collected by the exporter.

@jedel1043 already did the work of documenting which metrics are collected by the exporter, so we just need to import it into MyST and publish here.