[FEATURE] Create map key

[sashamaryl]

create a key for the map (see figma for design, use d3)

include:

• color gradient block scale
• symbol guide

should be able to accommodate other types of scales and additional data
should be reusable for other types of visualizations (not tied to map-type)

[edward-malouf]

Is this enough of a gradient? I think we might want to go darker by two additional steps.