Create documentation for fundamental geospatial concepts for cesium-native

[j9liu]

This is copied from a post on our community forum soliciting feedback from our users about documentation.

More Geospatial learning resources, I feel like we all could benefit a lot from including more sources and recommended readings for the developers in Geospatial topics, this is not so much a topic that relates directly to documentation, but I believe many devs will be interested in learning more about the foundational principles of the technology they’re implementing and its use cases when these resources are available within the documentation.

We should start identifying the fundamental concepts that get used in cesium-native -- e.g., the ellipsoid model, ECEF. We can begin with simpler introductory materials, and then -- if we see a need -- add more advanced resources over time, or at least create a reading list for those advanced topics.