docs(studio): document de locale availability

[hendrikheil]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Documents the availability of a de locale in Studio. Relies on nuxt-content/studio#119

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[vercel]

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[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/content@3618

commit: ac97643