Gallery example for symbol magnitude legend

[maxrjones]

Description of the desired feature

Someone in the AGU workshop today asked how to show the legend for the magnitude scale in response to the geopandas tutorial. It would be great to add this as a gallery example. https://github.com/michaelgrund/GMT-plotting/blob/main/009_paper_GR2020/pygmt_jn_fig_2/GR_2020_Fig_2.ipynb shows one approach.

Are you willing to help implement and maintain this feature?

Maybe

[seisman]

Done by #4214.