Move back doctest instructions to setup.cfg
# What does this PR do?

As a result of #22539, the options we have for the doctests are now all ignored. This PR reverts the change for those and puts them back in `setup.cfg`.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22587). All of your documentation changes will be reflected on that endpoint.