Add examples with illustrations to the -g cookbook documentation

[maxrjones]

Description of the desired feature

The documentation for the -g common option in the cookbook (https://docs.generic-mapping-tools.org/dev/cookbook/options.html#data-gap-detection-the-g-option) is relatively terse.

Additional examples with illustrations should be added to that cookbook section, similar to the examples provided for the -w option (https://docs.generic-mapping-tools.org/dev/cookbook/options.html#examining-data-cycles-the-w-option).

Originally posted by @PaulWessel in this forum post.