[Documentation] Show how to gather the data from cache

[jan-janssen]

Example:

import os
import pandas
import shutil
from executorlib import Executor
from executorlib.standalone.hdf import get_cache_data

cache_directory = "./cache"
with Executor(backend="local", cache_directory=cache_directory) as exe:
    future_lst = [exe.submit(sum, [i, i]) for i in range(1, 4)]
    print([f.result() for f in future_lst])

df = pandas.DataFrame(get_cache_data(cache_directory=cache_directory))
df

This was implemented in #525