Add docs for @carbon/icons-react usage in @carbon/react to Storybook

[joshblack]

We are providing a new export scheme with @carbon/react that allows teams to import icons directly from @carbon/icons-react. We should document this as a new section and set of stories/docs in the Storybook for @carbon/react. This documentation should include:

• Creating a new section outside of Components And Welcome, potentially elements?, that includes the stories for Icon
• Write a story using MDX which details how to bring in icons using this new import path