docs-bug(chips): the "form example" is not fully related to forms

### Documentation Feedback

We have `chips-form-control-example`, `chips-reactive-form-example` components that intend to illustrate how chips interact with the form control.
I see 2 problems here.

1. The interaction with `mat-chip-grid` is happening not fully through the formControl. For now the source of chips's list is `keywords` signal, not the formControl: `@for (keyword of keywords(); track keyword) {`
It seems we should take the formControl as the source of chips's list (not the signal): `@for (keyword of formControl.value || []; track keyword) {`
Moreover we could get rid of the signal, because the goal of the example is to show the interaction between the formControl and the chips in the template.

2. These two examples of `chips-form-control-example` and `chips-reactive-form-example` components duplicate each other. They have the same logic inside of components classes. So it seems we could leave only one of these two examples.

### Affected documentation page

https://material.angular.dev/components/chips/examples

issueで参照されている chips-form-control-example コンポーネントと chips-reactive-form-example コンポーネントから始め、chips のドキュメントページに示されている例を確認します。それらのクラスとテンプレートを比較します。完了条件は、chips リストが keywords signal ではなく formControl を通じて示され、重複した例が削除または統合されていることです。

索引モデルが issue の本文から書いたものです。