Worker implementation guidelines

Originally described in https://gitlab.com/teklia/arkindex/doc/-/issues/17 but now this kind of documentation is written here.

From https://gitlab.com/teklia/arkindex/doc/-/issues/4: Using (partly) this page from the wiki, to give guidelines on writing a worker.

No need to go too much in depth as we are not going to write a tutorial on how to do Machine Learning or how to write in Python. Try to generalize further than just machine learning, since workers can be anything, not just ML workers.