[Documentation] Explain how to access the executorlib_worker_id for debugging

The functionality was added in: https://github.com/pyiron/executorlib/pull/748

Basically, the worker ID is available as input parameter to your functions named executorlib_worker_id, so if you have a function like this:

def my_function(a, b, executorlib_worker_id): 
    ...

And you submit it to an executor like this:

exe.submit(my_function, a=1, b=2)

Then executorlib automatically sets the executorlib_worker_id input parameter at the point when the function is executed on the worker, in analogy to the init_function: https://executorlib.readthedocs.io/en/latest/1-single-node.html#performance-optimization

Start with the existing single-node performance-optimization documentation and the functionality introduced by pull request #748. Add an example showing executorlib_worker_id in a submitted function and explain that the executor supplies it when the function runs; the documentation should make the debugging workflow clear.