Create a prek hook to automatically export the module structure with docstrings to a tree diagram file #1576

Toward #1574.

The hook would give an ASCII tree diagram to a configurable output location specified e.g. via args: ["--output-file=docs/module-tree.txt"].
It would only include .py files. __init__.py files would be handled specially and their module docstrings would be added against the directory/module they are associated with.

next to tree entries would be a # comment giving the first line of the module docstring.

The tree diagram would be based on module names, not based on filepaths. So no .py suffixes, etc.