reintroduce ClusterManagers example

[wikfeldt]

from clusters.rst (currently replaced by hpc-cluster.rst)

[code4yonglei]

The usage of ClusterManagers.jl is cluster-dependent and it is not so easy to handle it. I have provided an additional session in the at https://enccs.github.io/julia-for-hpc/hpc-cluster/#running-julia-application-in-a-job so that the person can come to these links for details if someone is interested to use this package.

[code4yonglei]

I will take time to see how to use it after the workshop and then test it on either LUMI or LB clusters.

[jaantollander]

ClusterManager.jl on LUMI should work.
I could add it to the HPC cluster section.
Here are some instructions: https://docs.csc.fi/support/tutorials/julia/#multi-processing-on-multiple-nodes

[jaantollander]

I added SlurmManager exercise about ClusterManager.jl to HPC section: https://enccs.github.io/julia-for-hpc/hpc-cluster/#exercises

I still need to test that everything works as expected.