[DOCTEST] Refactor doctest for simplicity and safety
# What does this PR do?
This a a draft PR to simplify the testing of documentation, which would rely on the `doctest` API. 
The documentation tests related to any model added will also be run as part of the CI that are not slow, juste to make sure that we fix everything in one go. (and that the contributors have an easier time doing it). 
I am also learning about github workflows and CI jobs, which is a good exercice! 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21152). All of your documentation changes will be reflected on that endpoint.