Start a map-over tutorial

[bernt-matthias]

Here I started a tutorial o how Galaxy processes dataset(s). I think it's important that users understand. It also should describe how to map over for tools that process multiple datasets at once.

As far as I can see this was missing so far.

In the near future I won't be able to actively work on this. But I would be super happy about any help that I could merge here :)

We could also think to integrate this here: https://training.galaxyproject.org/training-material/topics/galaxy-interface/tutorials/collections/tutorial.html .. but to me it seemed not to fit well.