Improve using values in composite workflows documentation

[olevski]

I had to spend ~20 mins yesterday to understand how to use a values file and specify parameters for composite workflows. I think this is too much of a barrier for user coming to renku for the first time or trying to use a new renku feature.

The docs for regular workflows are not super clear either. But there are a few sentences in the docs that help.

But how to properly specify the proper nested workflow name in the values file is not clear to me. Especially if you are looking at the CWL export of a composite workflow. I was only able to get this to work after trying a few different names/ids.