[documentation] Add an example with a for-loop - to demonstrate how to accelerate a workflow

[jan-janssen]

Currently, theresult() call right after thesubmit() gives the impression that both lines should follow each other.

[jan-janssen]

import flux.job
from pympipool import Executor

with flux.job.FluxExecutor() as flux_exe:
    with Executor(max_cores=1, executor=flux_exe) as exe:
        future_lst = [exe.submit(sum, [i,i]) for i in range(10)]
        print([future.result() for future in future_lst])

[jan-janssen]

The very first example of the documentation now covers the for loop: https://executorlib.readthedocs.io/en/latest/README.html#examples