Add Spanish translation of serialization.mdx
# What does this PR do?

Add the Spanish translation for `serialization.mdx` as part of the #15947 issue.
Changes include the Spanish version of the original document and the updated `_toctree.yml` file.


## 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?
- [x] Was this discussed/approved via a Github issue or the [forum](https://github.com/huggingface/transformers/issues/15947#issuecomment-1312741421)**.
- [x] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@omarespejel or @osanseviero, can you help me review this PR? Thanks!<|||||>@osanseviero, I addressed your comments in my last commit. Thanks for your review. I'll consider your feedback for future translations.<|||||>@sgugger, review done :) Please merge when possible. Thanks!