docs: clarify that definePageMeta only works in the pages directory

[donatelloraphael]

Added warning about definePageMeta macro only working in pages in pages/ directory

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• 📖 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)

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📝 Checklist

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

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[danielroe]

Perhaps just at the start of the section?

Within your pages/ directory, you can use definePageMeta along with useHead to set metadata based on the current route.

And then add a link at the end to https://v3.nuxtjs.org/guide/directory-structure/pages/#page-metadata?

[pi0]

@danielroe Do you mind pushing changes?