Learning material – Geographic coordinates guide

We get many questions on the community forum around coordinate systems, enough that it probably warrants its own guide.

This would build on top of a [more general geospatial guide](https://github.com/CesiumGS/cesium-native/issues/980) which we hope to ship soon, but focus on the CesiumJS API.

1. What coordinate system(s) does CesiumJS use: `CartesianX`, `Cartographic`, degrees vs radians
2. How to convert between them
3. Any generally helpful tips, like the `Transforms` class, or East-North-Up as a reference frame
4. Maybe: MSL vs Ellipsoid height

Start with the linked general geospatial guide issue and the CesiumJS API material for CartesianX, Cartographic, Transforms, and East-North-Up. The guide is done when it explains the coordinate systems, degrees versus radians, conversions, reference frames, and optionally MSL versus ellipsoid height for CesiumJS users.

Written by the indexing model from the issue text.