docs: add multiple slots example

[RollingTL]

❓ Type of change

• [x] 📖 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

It is not very clear in current docs how to use multiple slots with ContentDoc

📝 Checklist

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