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

从“Creating a Custom Form Field Control”指南开始，重点查看 issue 中链接的“Form field with custom telephone number input control”示例。检查现有的 Signals 和 Signal effects 用法，然后更新指南的其余部分，使其中的说明和示例始终使用 Signals；指南应在全文中清楚地讲解当前基于 Signals 的方法。

由索引模型根据 Issue 内容生成。