docs: add note and banner about edge channel

[pi0]

🔗 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

• Add a dedicated page explaining about opting-in and opting out from edge channel.
• Add a badge to indicate features from the documentation waiting for next release with possibility of beta testing them earlier.
• Add note for optional params (resolvesoptional route param naming ([[param]]) don't work as documented nuxt#13846)

📝 Checklist

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

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[pi0]

@danielroe Merge earlier to make users aware of this. Feel free to make an PR improving this.