docs: add extend pages page

[BobbieGoede]

🔗 Linked issue

#1894

❓ 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

The added page describes how a module author can add pages from a module and how to make sure the routes of these pages are localized.

📝 Checklist

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

[kazupon]

Thank you so much! :)