feat: support navigation field in content and _dir.yml

[atinux]

🔗 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

This PR adds support for a navigation field in both pages and _dir.yml to overwrite the data. This also remove the extra step to add keys into navigation.fields in the nuxt.config.

---
navigation.title: 'Home'
---

# Another title

Also, it fixes a bug where the title and description are not discovered with empty front-matter:

---
---

# Title not detected

Description not detected as well

📝 Checklist

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

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