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

Commencez par le guide “Creating a Custom Form Field Control”, en particulier par l’exemple “Form field with custom telephone number input control” associé dans l’issue. Examinez l’utilisation existante de Signals et de Signal effects, puis mettez à jour le reste du guide afin que ses explications et ses exemples utilisent systématiquement Signals ; le guide doit clairement présenter l’approche actuelle basée sur Signals dans son ensemble.

Rédigé par le modèle d'indexation à partir du texte de l'issue.