Add map legend for all overlays on taxon pages

I’d like to repeat @jabrams_foc 's question in https://forum.inaturalist.org/t/map-legend-what-does-the-orange-color-represent/3573 as a feature request: “could [we] get a clear legend for the overlays on species maps?” A legend on the map itself would be a nice improvement over simply documenting the answer to this question in the wiki.