Add simple example to `user_workflow_vignette.Rmd`

Currently this vignettes does not contain a working example to show how r2ogs6can be used to define, run and retrieve results step-by-step.

changed milestone to %stable version 0.1 for MA-thesis Anna

added docs label

mentioned in merge request !21 (merged)

closed with merge request !21 (merged)

mentioned in commit c50f4b7d