Produces a fixed set of hero SVGs used by the generated documentation to illustrate each plot type.
The output is committed to pages/assets/. ExDoc's :assets option
copies that directory into the generated doc/ tree so 
markdown references resolve from both guide pages and module docs.
Re-run after any rendering change that should be reflected in the docs:
mix bland.docs.examplesEvery example is deterministic — the RNG is seeded so the images don't churn between commits.