RadioGroup: Adjustment to visual checkbox small size

[benjamincharity]

This change is needed for LinkedIn Forms.

Currently, the 'small' visual mode simply a smaller responsive ratio. Moving forward, UX would like more control over the display of the visual checkbox.

⚠️ NOTE: This ticket should only affect the small visual radio.

• Remove the responsive mixin and hardcode the width to 13.75rem
• The height should also be hardcoded to a height using rems that allows for:
  • 2 title lines
  • 3 body lines
  • The appropriate top/bottom padding
• Update the component JSDoc comment to include all possible inputs & outputs
• Update usage docs to include instructions about how to use the 'small' visual mode

[dpeet]

Related problem (that may need to be it's own issue) is adding the shadow mixin to all of the radio buttons so they are more "button" like

[benjamincharity]

@dpeet I went ahead and popped that off into it's own issue #1574