[Documentation] Setting queuing system priority

[jan-janssen]

• For the FluxJobExecutor this functionality was already implemented in [Feature] Implement flux job scheduling priorities #592
• For the SlurmJobExecutor this could be done by setting the --nice value https://slurm.schedmd.com/srun.html
• For the SlurmClusterExecutor in analogy to the SlurmJobExecutor this can again be achieved by adding the --nice value in the submission template https://executorlib.readthedocs.io/en/latest/2-hpc-cluster.html#slurm and then providing it in the run template.
• For the FluxClusterExecutor this should be done as well.