[i18n-es] Translating docs to spanish

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What does this PR do?

Add the Spanish version of index.md, _toctree.yml, installation.md to smolagents/docs/source/es.

Before reviewing

• Check for missing / redundant translations
• Grammar Check
• Review or Add new terms to glossary
• Check Inline TOC (e.g. [[lowercased-header]])
• Check live-preview for gotchas

Who can review? (Initial)

May you please review this PR?
@aaronjimv @weezymatt

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? (Final)
@albertvillanova

[aaronjimv]

Hi!

I have no comments regarding the translation. I find it fluid and well-written, great work!

It only remains to verify that the structure and format of the files are the same to the English documentation, since this is the first Spanish documentation in the library. (Just in case)

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Thank you @aaronjimv!