Examples changes that were necessary:
- loft.py: failed on macos (only) because of (seemingly) over-precise
floating-point accuracy assertion. Loosened the tolerance, and
expressed it as a multiple of the expected value.
> AssertionError: delta=0.002982314711971412 is greater than tolerance=0.001; got=1306.3375467197516, want=1306.3405290344635
- packed_boxes.py: only emit output files when GEN_DOCS is
True (mimicking lego.py).
Also added documentation for the new facility as well as a nascent
CONTRIBUTING.md to collect pointers to the tools used by developers of
this project.
