Update Sandcastle examples to match most common use cases

From discussion related to #4081

It would be helpful to our users if our sandcastle examples more closely resembled scenarios for which they would be using Cesium. This is particularly true for geometry examples. Users will likely not make polygons that are huge and take up most of the united states. We should try to make the demos mimic real-world use cases.

Review the existing Sandcastle examples, especially the geometry demos, and read the discussion in #4081 for context. Update the examples so they represent common, real-world Cesium use cases rather than oversized polygons; done means the demos better match how users are likely to use Cesium.

Written by the indexing model from the issue text.