docs-bug(MatButtonToggleGroup): document how to use it with reactive/template-driven forms

[AlexElin]

Documentation Feedback

I think there's sense to document how to use MatButtonToggleGroup with reactive/template-driven forms.
This could be done similar to mat-slide-toggle: a note in Overview + Example

I see MatButtonToggleGroup implements ControlValueAccessor so it can be used with forms. But it should be documented

Affected documentation page: https://material.angular.io/components/button-toggle/overview

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