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 (resolves optional 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.

[netlify]

✅ Deploy Preview for nuxt3-docs ready!

Name	Link
🔨 Latest commit	690fc71
🔍 Latest deploy log	https://app.netlify.com/sites/nuxt3-docs/deploys/6268366b2add99000909311d
😎 Deploy Preview	https://deploy-preview-4640--nuxt3-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[pi0]

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