fix: Add prop table for modal story

[nats12]

Short description

During the upgrade to Storybook 6 and the shift to MDX stories, we noticed that the prop types for the Modal story weren't inferred due to its structure (a component with subcomponents with their own props and types as its type).
After a lot of researching online and creating a discussion in Storybook's repo, we have yet to find a way to automatically infer prop types of this component. Two workarounds were: a) inferring prop types from one of the actual stories i.e. Playground and not the component or b) manually set these props and their descriptions (the former wouldn't handle this). In this PR, I've attempted option b.

Granted it may not be the best solution, as any changes made to the Modal component will mean inconsistent prop types between itself and the story. But I'm struggling to see how we can get these inferred without completely changing the structure of our Modal component so that it's no longer a component made up of types.

This PR can serve as a bit of an exploratory area if we're not happy with this approach. Feel free to suggest alternative ways to achieve this or even contribute to this branch.

PR Checklist

• Run storybook
• Visit the modal story and click on it's Docs tab
  • Make sure you see a prop table with all the same Modal props we see under the Control tab
  • Make sure all props under this table have descriptions
  • Make sure that changing the prop's controls changes the Playground story.

[frankieyan]

Using the Canvas instead of Preview seems to allow the right source code to be displayed in each example as well. Awesome!