fix(ui-checkbox): warn CheckboxGroup onChange

[buckett]

CheckboxGroup overrides the onChange value of contained Checkbox components. If a developer incorrectly sets an onChange property on the Checkbox contained within a CheckboxGroup we write a warning to the console in the standard way.

The behaviour is also added to the documentation in the "Guidelines -> Don't" section.

[CLAassistant]

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[matyasf]

Thanks for the contribution! We are looking into it