Add a build process for generating CSS files and instructions for how to use them

[bspeare]

Relates to https://github.com/uiowa/uids/issues/908.

How to test

• yarn production
• npx http-server ./www
• Go to the Introduction page and confirm that CSS files are listed using the version number from package.json.
• Go to the Brand Bar docs page and confirm that the CSS files are listed using the version number from package.json.

[pyrello]

I'm going to look at this again today.