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

Inizia con i componenti chips-form-control-example e chips-reactive-form-example menzionati nell’issue e analizza gli esempi mostrati nella pagina della documentazione di chips. Confronta le relative classi e i template; il lavoro è completato quando l’elenco di chips viene mostrato tramite il formControl invece del keywords signal e l’esempio duplicato viene rimosso o consolidato.

Scritto dal modello di indicizzazione a partire dal testo della issue.