Write documentation for how multithreading is handled in cesium-native

[nithinp7]

Once the clipping PR goes into main, the ./doc/threading.md document will have been deleted. Was it out-of-date or was the removal otherwise necessary for some reason? For reference here is a permalink to the original threading.md: https://github.com/CesiumGS/cesium-native/blob/580beec2190755b9e5871d3d70d6d9f2b71f79e8/doc/threading.md

[kring]

The old threading.md was really old and completely wrong. We really should have an updated version, though, so I'll change this issue to request that.