chore(docs): add CodeSandbox template example

[austingreendev]

Description

This PR adds documentation linking to a new CodeSandbox template: https://codesandbox.io/s/1qw90o82pl

Please take a look and let me know if there is anything else that should be added. It is currently using create-react-app@v2 so everything should be as up-to-date as possible.

Detail

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 95.833% when pulling 23db9e1 on agreen/codesandbox-example into 3ad4dcc on master.

[ryanseddon]

Hooray for codesandbox! We should definitely make this into a kitchensink example and just include all the the things.

[austingreendev]

We should definitely make this into a kitchensink example and just include all the the things.

I've updated the code example to include all of our current component packages. I've also pre-added the scoped styles.css for each in the global-styles.js file.