i18n(fr): create `astro-courses.mdx`

[ArmandPhilippot]

Description (required)

Translates in French the new astro-courses page added in #11895

Related issues & labels (optional)

• Suggested label: i18n

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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.