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

Start with the “Creating a Custom Form Field Control” guide, especially the “Form field with custom telephone number input control” example linked in the issue. Review the existing Signals and Signal effects usage, then update the rest of the guide so its explanations and examples consistently use Signals; the guide should clearly teach the current Signals-based approach throughout.

Written by the indexing model from the issue text.