[i18n-ar] Translated file : `docs/source/ar/trainer.md` into Arabic

[AhmedAlmaghz]

What does this PR do?

Translated the docs/source/ar/trainer.md file of the documentation to Arabic العربية. I use transformers for my side-project and wanted to contribute. As a beginner, this seemed appropriate.

Some questions:

• I can't access the pr documentation endpoint for Arabic العربية. Is there a missing step?
• I left all the other files alone after copying the English docs. Is this acceptable for future updates or should I add contents to the _toctree.yml only when translation is complete? Should I delete all other files for a cleaner review?
• In Arabic, people use both Transformer and المحولات to describe the model. However, as this is more of a product of HuggingFace I opted not to translate it. May you please let me know which you would prefer?

Thank you in advance for your review.

Part of #32435

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum ?Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

@stevhliu , @MKhalusova , @abodacs, may you please review this PR?