i18n(fr): add backend/scalekit.mdx & update guides/authentication.mdx

[ArmandPhilippot]

Description (required)

Adds changes from #13465 to the French translations:

• translate the new backend/scalekit.mdx guide
• update guides/authentication.mdx

Related issues & labels (optional)

• Suggested label: i18n

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Tracked Files

File	Note
fr/guides/authentication.mdx	Localization changed, will be marked as complete.
fr/guides/backend/scalekit.mdx	Localization added, 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.