Add code view to example pages

[nateplusplus]

Example pages like https://pushinjs.com/examples/simple/ could use an easy way to see the source code, so that they can be used as code examples.

• Add sub-navigation to example pages
• Options for subnav: Result, Vanilla, React
  • Clicking on Result should show the example as a demo
  • Clicking on Vanilla should show plain HTML and vanilla JS code used to create the example
  • Clicking on React should show a React code snippet that can be used to create the example

[nateplusplus]

Worth consideration: https://developer.stackblitz.com/platform/api/javascript-sdk

I'm not sure if it's exactly what we're after here.