Breakup `example.md` into smaller clearer examples

[rofinn]

We currently have a big jump from the docstring examples in the API docs and the one very large example that assumes you're already familiar with the API. This documentation structure results in a lot of cognitive load when learning the package, particularly since it pulls in syntax from half a dozen other packages (e.g., Tables, DataFrames, AxisKeys)

[rofinn]

I'm also wondering if a diagram like this would help folks visualize what's happening.