Add a Basemap page for RSContext

[joewheaton]

From #29 from @stephen-bennett we successfully added Basemaps for Geology to QRAVE (doesn't work well for Arc and basemaps don't work in WebRAVE for obvious reasons).

We need:

• a page on the https://tools.riverscapes.xyz/rscontext/ documentation that explains what basemaps are, what are the differences between how they work (and don't) in QRAVE, ArcRAVE and WebRAVE, and why.
• a part of the page that lists each basemap we've added to RAVE, where it came from, what it is, and an example of it.
• a standard for referencing these layers from RAVE (i.e. right click on basemap layers and it takes you to this part of documentation page).