Modal: component and documentation update

[Spark450]

Acceptance Criteria:

• max-width not working sandbox property not currently working, this should work as expected
  • set default max width of 30rem for default modal
  • set default max width of 33rem for callout modal (do we need this to be different or can it just be 30 for both?)
  • Add the following helper text to the max-width input: "Enter width with unit (px,vw,rem,em,ch)"
• Change sandbox input label from "variant" to "Callout variant" to match the property name
  • include a default "—Select—" option in the dropdown.
• Update all examples so that only action buttons display and not the "close" icon in the top right corner
• On mobile variant colored band should be displayed horizontally at the top of the modal
• Update the sandbox to use the "actions" slot