Add prop types and a document for a suggested approacj

I'd like to use proptypes to do basic ui testing for the app. I'm not as good with it so I'd love help setting that up.

The issue names no files, tests, or existing test configuration. Start by locating the app's UI components and current testing setup, then define the suggested PropTypes approach and document it; done means the setup is usable for basic UI testing and the approach is documented.

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