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

Inizia con la guida “Creating a Custom Form Field Control”, in particolare con l’esempio “Form field with custom telephone number input control” collegato nell’issue. Esamina l’uso esistente di Signals e Signal effects, quindi aggiorna il resto della guida in modo che le spiegazioni e gli esempi utilizzino Signals in modo coerente; la guida dovrebbe insegnare chiaramente l’approccio attuale basato su Signals in ogni sua parte.

Scritto dal modello di indicizzazione a partire dal testo della issue.