Generates @moduledoc documentation with an ASCII flowchart showing
all possible execution paths for every module that uses Hephaestus.Workflow.
$ mix hephaestus.gen.docs
The task compiles the project, discovers workflow modules via the
__graph__/0 function injected by the macro, and updates each source
file with an ASCII diagram rendered top-to-bottom.
Re-running the task is idempotent — it replaces the previously generated section between HTML comment markers.