Update Basemap.xml documentation to reflect new workflow of adding to all default basemaps.

[joewheaton]

http://rave.riverscapes.xyz/Technical_Reference/base-maps.html is out of date and does not show how to add to basemaps.xml for all.

See: Riverscapes/RiverscapesXML#415 and https://github.com/Riverscapes/riverscapes-website/discussions/29