doc: initial audio reactivity guide example

[sbaier1]

initial draft, not sure if video embedding will work in any way here...

closes #33

[dmarx]

Thanks so much!

This is a somewhat advanced tutorial: could you maybe add a "beginner" tutorial that focuses more on providing a step-by-step demonstration of how to build a simple animation that minimally takes advantage of audio-reactivity? Like, assume the user completely ignored the settings page and came to this tutorial without even knowing precisely what "audioreactivity" means. In addition to this excellent discussion of how to use supplemental tools and techniques to get the most out of this feature, could you do another tutorial that illustrates basic usage?

[sbaier1]

I added a basic explanation of the whole concept, how it works end-to-end and another very simple example that can be easily reproduced.