Add illustration to complicated module options

Hi @GenericMappingTools/gmt-contributors-

The documentation for [plot](https://docs.generic-mapping-tools.org/dev/plot.html) got much better after adding the illustrations of the many type of symbol groups. However, there are many other modules that could benefit from either

A representative figure just below the `Description` section
An illustrative figure at the end of the stuff for some option -X

Some may need both. This requires familiarity with GMT and ability and willingness to write a short script that makes the figure, or search through test/module to see if we already have one among the tests. If you have suggestions for such and willing to do it, open a PR with a title like "WIP supporting figure for module xxxxxx" where your branch off master simply adds the script with a useful name like the others in doc/script, e.g. GMT_segmentations.sh. Select the core collaborator team for the review. The team can then try it out. Just post the figure on GItHub for now (do not add PS file to DVC etc since we need to review first).

Ideally, the illustration is made at 15cm by 5 cm unless by nature it needs to be taller or square.