i18n(de): update islands.mdx

[louisescher]

Description (required)

This PR includes some wording changes according to the new consensus reached in #12500: Islands are now no longer translated as "Inseln" in the German translation.

This PR is meant to act as a draft for German contributors to check out this change before we merge the updated guidelines.

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Tracked Files

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de/concepts/islands.mdx	Localization changed, will be marked as complete.
de/guides/migrate-to-astro/index.mdx	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.

[trueberryless]

Schaut allgemein supi aus! 🙌

Ein paar Kleinigkeiten sind mir aufgefallen

[trueberryless]

Looks good to me know 🥳
We might want to retrigger the Netlify preview tho by updating the PR (should be fixed now) to see the changes live 👍

[yanthomasdev]

LGTM! Thanks, I went ahead and reverted the <h3> to ### change, it is supposed to be like that in the original to hide those headings from the "On this page"!