Fix docs code extraction, following mkdocstrings refacto

[percevalw]

Originally posted by @pawamoy in #359:

I see that you add your own extension to the config of mkdocstrings' autodoc processor:

edsnlp/tests/extract_docs_code.py

Line 95 in f14900d

ext._config["mdx"].append(self)

Note that following a refactor (work in progress), this attribute won't exist in the next version of mkdocstrings.

I'm not sure to understand how this code is used, and what for, so I'm not sure how to fix it. Just know that if you modify the global config's markdown_extensions list, our processor should pick it up. I'll let you know once there's a branch you can test on.