Add Multi-Build Usage Example

This is similar to https://github.com/intuit/postcss-themed/issues/23 and https://github.com/intuit/postcss-themed/issues/22!

This project has a niche option called [forceSingleTheme](https://github.com/intuit/postcss-themed#forcesingletheme), which only outputs one theme. We use this with DS-CLI [Multi Build](https://github.com/intuit/design-systems-cli/releases/tag/v2.3.0) to generate multiple theme files for a single component.

Basically:

- Create a component with two themes.
- Run PostCSS once to generate a file with multiple themes.
- Run PostCSS again to create a CSS file with only Theme1.
- Run PostCSS again to create a CSS file with only Theme2.

While we have this working on an internal project, we don't have any public examples of these two projects working together! If you are interested in learning more about DS-CLI or our theme architecture, this is the perfect project.

I would like to create a folder in the repository called `examples` and in it add a folder called `multibuild` which has a simple app using the modern theme configuration and PostCSS.

This would require:

- Creating the folders.
- Create a sample design system with a single component in it.
- Set up DS-CLI multi build to run multiple PostCSS configurations.
- Write some documentation to show the setup and have people inspect the generated output.