Update "How it Works" Guide

[deepsheth]

Is your feature request related to a problem? Please describe.
With the release of theme-ui v0.15, the How it Works guide looks slightly outdated.

Specifically the guide says

The Theme UI ThemeProvider component is a wrapper around MDX's MDXProvider, which adds custom React components to context

contradicts what's on the Migration Guide

MDXProvider is no longer included in Theme UI ThemeProvider, and has been removed in favour of an useThemedStylesWithMdx hook.

Describe the solution you'd like
Update the How it Works guide and the code snippet which shows the MDX Provider included inside the ThemeProvider.

[hasparus]

Good catch @deepsheth! Thanks for the issue.