Review vocabulary around workflows in the documentation

We should review the introduction and the tutorial (and maybe other pages) to try to try clarify the relationship between "Debusine Workflows" as in "Orchestration logic supported by Debusine as a software", "Workflow templates" as in "Configured workflows made available in a workspace" and "Workflows" as in "Running instance of a workflow".

We should come up with a proper terminology, and also proper verbs associated to each action:

• Going from "Debusine workflows" to a "Workflow template" could be named "Configuring a workflow"
• Going from "Workflow template" to a "Running workflow" could be named "Starting a workflow"

"Workflow template" is rather arcane, I wonder whether we should note aim to use "Configured workflows" in the documentation and user interface, and keep the WorkflowTemplate only for development.

(split off from #770 (closed), /cc @mbehrle)