Add custom create react app with flex-layout included

Given the amount of times we need to do it manually, I think it might be worthy to create a custom create react app template to generate a react app with geppetto's flex-layout incorporated.

https://stackoverflow.com/c/metacell/questions/67/68#68
https://create-react-app.dev/docs/custom-templates/

Start with the linked Create React App custom templates documentation and the referenced Stack Overflow discussion. Define how Geppetto's flex-layout should be incorporated into the generated React app and where the template should live. Done means the template can generate a React app with flex-layout included and the setup process no longer requires the repeated manual steps.

Written by the indexing model from the issue text.