Enable toggle of code snippets in docs-demo

This is a feature suggestion to enable toggle of code snippets. This would be useful in cases where ember-cli-addon-docs is used to document UI-kit addons. The ability to only see code snippets when needed would help make this type of documentation more readable.

video example: https://imgur.com/uLaRiSP
example: https://semantic-ui.com/elements/button.html