Improve docs: how to actually add and show a video

[mjmare]

It took me some time to find out that adding a "Video" content item is not enough to show a video (say by embedding a link in the texture). One has to add "components" like an mp4 file, a placeholder image etc.

As a naive user I expected to be able to embed Youtube or Vimeo videos as well. But that seems not to be possible.

Thanks!