Update docs/README.md
# What does this PR do?

Add information about how to perform doc testing **for a specific python module**. Potentially making the process less scaring (a lot of file changes) for the users. 

Also make it more clear/transparent that `file_utils.py` should be (almost) always included.

_The documentation is not available anymore as the PR was closed or merged._<|||||>Do you mean: there are some python module files or doc files, for which if we want to run doctest, it will require changes (preparation) in files other than 

- the target files themselves
-  `file_utils.py`
<|||||>> Ah sorry, I misread the command! This works indeed but could we maybe wait for the refactor of `file_utils` to merge this (and fix the paths)? I'll probably forget to adapt this otherwise.

Oh, yes for sure!<|||||>Will merge this PR once Sylvain's PR about file_utils is merged.