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

“Creating a Custom Form Field Control”ガイドから始め、特にissueでリンクされている“Form field with custom telephone number input control”の例を確認してください。既存のSignalsとSignal effectsの使用状況を確認したうえで、ガイドの残りの部分を更新し、説明と例で一貫してSignalsを使用するようにしてください。ガイド全体で、現在のSignalsベースのアプローチを明確に説明する必要があります。

索引モデルが issue の本文から書いたものです。