DOCS: Add Slurm example

Add an example on how to request a multi-host ad-hoc cluster by requesting it via Slurm. This involves spinning up workers using `srun`.

No documentation file or test is named. First locate the existing documentation and examples for cluster setup, then review how workers are started with srun and how multi-host ad-hoc clusters are requested through Slurm. Done means the documentation contains a runnable, clearly explained multi-host example.

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