i18n(fr): update guides/server-islands.mdx

[ArmandPhilippot]

Description (required)

Updates the French translation of guides/server-islands.mdx:

• add changes from Improve documentation about ASTRO_KEY #11751
• update the wording to match the i18n guide see i18n(fr): update the French i18n-guide #11593 (+ consistent translation for fallback, we had de secours/de repli)

Related issues & labels (optional)

• Suggested label: i18n

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fr/guides/server-islands.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.

[dreyfus92]

LGTM ✅