Examples for radio and checkbox toggled content

Add javascript for radio button and checkbox examples:
* this now shows and hides content with an ID matching the `data-target` attribute 
* `aria-controls` and `aria-expanded` attributes are set on the input, which controls the hidden content 
* the `aria-hidden` attribute is set on the toggled panel

Add new checkbox example showing Register to vote - What is your
nationality?

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