Create Icon radio button and icon checkbox documentation

[blackbaud-conorwright]

Tied to PR: #1892

The inputs have been added:

• icon: string = What font awesome icon should be used in the icon button. Examples: umbrella, bold, or ban. If the icon input is set, the button will display an icon button instead of the standard button.
• checkboxType/radioType: string = What type/coloring of button to make. Shares same naming and colors as the alert component. Available values are: info(default), success, warning, and danger

There is also a new sky-switch-icon-group class that can be placed on the direct parent element of the buttons to group them next to each other. There is an example of its use in the demo.

[blackbaud-johnly]

Addressed in blackbaud/skyux2-docs#200.