Add Spanish translation of custom_models.mdx
Author: donelianc
Pull request: yes

# What does this PR do?

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Add the Spanish translation for `custom_models.mdx` as part of the #15947 issue. 
Changes include the Spanish version of the original document and the updated `_toctree.yml` file.

Tried to follow the [guideline](https://github.com/huggingface/transformers/blob/26a6a426087582c48593f8be980603951a7bcddd/CONTRIBUTING.md#start-contributing-pull-requests) to generate the Markdown files and check them before submitting this PR but the command `doc-builder build transformers docs/source/ --build_dir ~/tmp/test-build` fails since the `_toctree.yml` file is no longer in `./docs/source/` (but in `./docs/source/en/`).

First contribution to the 🤗 Transformers project!

## 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://discuss.huggingface.co/)? Task assignment [here](https://github.com/huggingface/transformers/issues/15947#issuecomment-1161854258).
- [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).
- [ ] ~~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.

Tagging @omarespejel, @osanseviero, or @sgugger to review or assign reviewers :)

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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@omarespejel could you take a look at this PR Please?

Hola @donelianc! Thank you very much for your translation. Sorry for the delay in replying; it won't happen again.

I made some small comments to be applied.

Thank you!

@omarespejel I committed the suggested changes :) If you don't mind, can you assign me the translation of [run_scripts.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/run_scripts.mdx)? I'll be happy to help with one more doc.