[documentation] Remove ShellExecutor and SubprocessExecutor from documentation

[jan-janssen]

The corresponding code was removed in #440 . For users who need this functionality the documentation should explain how to interact with an external executable based on the updated unit tests.

[jan-janssen]

Done. In addition, I added examples how to implement the same functionality in the developer section of the readme:
https://executorlib.readthedocs.io/en/latest/4-developer.html