Update seng-generator mentions on docs

[skulptur]

We include seng-generator by default as a dependency now, and we added the command yarn generate for sg wizard. That means devs don't need to install seng-generator globally anymore, and in the docs we can and should instruct using yarn generate whenever applicable.

https://github.com/mediamonks/muban/blob/master/docs/02-setup-guide.md#install-the-seng-generator-cli