Guide: Running an app

[avaer]

• [ ] Add a textual guide for opening Exokit and running various apps
• [x] Add a video going through the steps

[avaer]

Replaces http://docs.webmr.io/docs/useExokitEngine.html

[avaer]

Good demo video links:

https://aframe.io/a-painter
https://supermedium.com/craft

[Fierent]

Sent video draft transfer.sh link

[avaer]

Fixed with https://github.com/webmixedreality/webmr-docs/pull/79.