[AI] Mark AISkeleton components stable

[tay1orjones]

Follow the established "move to stable" checklist:

• All files have a copyright banner
• All components exported in src/index.js and should not be unstable_
  prefixed
• Component has a label in the GitHub repository
• Component should be documented on the website
  • Component should have a usage, style, and code tab
  • Component may have a component demo
• For each component exported:
  • Component is written as a function declaration or uses forwardRef
  • Component has propTypes defined
    • Each prop type has a comment (used in storybook)
    • Prop types are as specific as needed, prefer PropTypes.shape over
      PropTypes.object if possible
  • Default props are listed as default args in the function definition (not
    in defaultProps)
    • Note: default props should be stable, in other words props like
      onClick = () => {} can cause re-renders since the function identity
      is not stable
  • Component has a story in <ComponentName>.stories.js
    • Component has an mdx document that follows our outline
    • mdx document coverages at least common use-cases and provides a prop
      table
    • Stories cover at least common use-cases
    • Stories may include a Playground story for controls
      • Controls with no meaningful change to the component visuals should
        be hidden from the controls panel, eg. className
      • Props of type node with no proper controls available for
        configuration should be hidden from the controls panel, eg.
        children
    • Stories should mirror intended usage of the component
  • Component has unit/integration tests written in RTL for testing the
    component API
  • Component is tested via VRT for at least the initial render state
  • Component is tested via AVT for at least the initial render state