Syntax chapter: add quick explanation of using workspace

[cdlm]

The first chapter talks about selecting code etc so it should show how to do it concretely, so that people can try the examples while they read.

[Ducasse]

I would point to a video. I'm fedup writing this into text and we already did it in Pharo by example. There are the videos of the Pharo mooc and we should use them. It takes far too much time to explain interaction.