Update Custom Field Guide for Payload 3.0 Compatibility

[Figumari]

Hello,

First off, thank you for this fantastic repository and the wonderful example!

I recently tried implementing the color picker example, but it seems that the current version isn't fully compatible with Payload CMS 3.0. Some imports (e.g., payload/components/forms and payload/components/preferences) and other APIs seem to have been restructured in the latest version.

Would it be possible to update this example to work seamlessly with Payload 3.0? It would be amazing to have a fully working and modern version of the color picker.

Thanks for your hard work on this guide and Payload! 🙏

Best regards, Figumari