Add terrain data

[wrynearson]

In #20, I moved away from Mapbox in support of MapLibre and OpenFreeMap. In doing so, access to Mapbox's terrain tiles (for the 3D map rendering) was lost. I don't think that was a critical feature.

See #20 (comment) for some ideas on how to implement this. I could generate terrain and hillshade tiles myself and host them here, or use this approach. If those don't work/are too much effort, I can add terrain tiles from Maptiler.