docs: add markdown.build.contentHeading option

[maximepvrt]

Add documentation for configuration option.

🔗 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

📝 Checklist

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

[vercel]

@maximepvrt is attempting to deploy a commit to the Nuxt Team on Vercel.

A member of the Team first needs to authorize it.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/content@3612

commit: 1dbb230