Add design blueprint for workflows

I restricted explanation/concepts.rst to the high level view of what workflows are and I pushed all the gory details in a new page in a new section called "development blueprints".

The idea is to put there all the plans for things that are not yet implemented so that we don't have a mix of things which are implemented and not implemented in the official documentation.

When the feature actually gets implemented, we should likely spread the useful bits in new "explanation" or "reference" pages and then drop the blueprint.