types: markdown option should be optional

[farnabaz]

🔗 Linked issue

resolves #3426

❓ 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

📝 Checklist

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

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Name	Status	Preview	Updated (UTC)
content-docs	✅ Ready (Inspect)	Visit Preview	Jun 24, 2025 1:45pm

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npm i https://pkg.pr.new/@nuxt/content@3427

commit: 58504dd

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