docs-bug(MatFormFieldControl): Update "Creating a Custom Form Field Control" for Signals

### Documentation Feedback

This is probably already on your TODO list, but in [this guide](https://material.angular.io/guide/creating-a-custom-form-field-control), the first example ("Form field with custom telephone number input control.") has a typescript file using Signals and Signal effects, but the rest of the guide does not mention Signals. It would be great to have the guide re-written using Signals. Creating a custom form field control can already be quite complicated, and Signals are a new concept, so having these docs up to date would be very helpful. Thanks!

### Affected documentation page

https://material.angular.io/guide/creating-a-custom-form-field-control

Comienza con la guía “Creating a Custom Form Field Control”, especialmente con el ejemplo “Form field with custom telephone number input control” enlazado en el issue. Revisa el uso existente de Signals y Signal effects y, a continuación, actualiza el resto de la guía para que sus explicaciones y ejemplos utilicen Signals de forma coherente; la guía debe enseñar claramente el enfoque actual basado en Signals en todo momento.

Escrito por el modelo de indexación a partir del texto del issue.