content: Dark Mode codepens should be replaced with full playgrounds

[averyjohnston]

URL

https://ionicframework.com/docs/theming/dark-mode

Issue Description

This page contains a couple of Codepen demos, which only showcase Ionic in vanilla JS. We should replace them (and perhaps some of the other code snippets on the page) with playgrounds/live demos like those found on the component docs (example) so that we can easily include code for Angular, React, and Vue as well.

There is a PR at #1648 that includes equivalent code for Angular and React.