Ensure all unnumbered docs examples are image-comp. tested

[sadielbartholomew]

First connected items of follow-on work from PR #81 (lifted from my opening comment):

• check for unnumbered examples across the docs and add all of these to the test suite, then ensure they work and coordinate so the precise working versions are documented;
• add reference plots to compare with, the uncomment the @compare_plot_results decorators.