i18n(es): update actions.mdx, add missing keys and review why-astro.mdx

[jramma]

I saw Sarah’s message on Discord after finishing this, and I didn’t realize v6 was so close. In this PR, I only reviewed what was outdated here: https://i18n.docs.astro.build/ — it’s nothing important. You can just dismiss the PR. I’ll try to contribute again once v6 is ready.

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File	Note
es/concepts/why-astro.mdx	Localization changed, will be marked as complete.
es/guides/actions.mdx	Localization changed, will be marked as complete.
src/content/i18n/es.yml	Localization changed, will be marked as complete.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

Hey @jramma ! Thank you for this PR, and I think it's small enough (and not involving pages that will change a lot in v6) that we can keep it... as long as we can get a Spanish reviewer quickly, hopefully! Would be great if you could ask in the es language thread and see if someone's available to quickly review, and we'll sneak it it! 🙌