Spanish translation of multiple_choice.mdx, question_answering.mdx.
# What does this PR do?

Translates `multiple_choice.mdx` and `question_answering.mdx` into Spanish. Also updates the `_toctree.yml` file accordingly. 

Fixes #15947

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?

@osanseviero @sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks @osanseviero, I just updated all files to reflect your suggestions :) <|||||>Thanks a lot! :fire: <|||||>@sgugger thanks for the merge! However, shouldn't the original issue (#15947) addressing the translations remain open? Think it was closed automatically bc I referenced it here.<|||||>Yes, that's because you used the word "Fixes". You should have said something like "Related to ..." :-)<|||||>Oops, sorry! Lesson learned :')