Document examples of aggregation and filtering

We'd like examples for each of the types of aggregation and filtering. We'd like to see what a schema and configuration might look like, and what you'd get as the result based on some input data.

Here's the existing documentation:
https://github.com/gchq/sleeper/blob/develop/docs/usage/data-processing.md
https://github.com/gchq/sleeper/blob/develop/docs/usage/properties/table/data_definition.md

The data processing document has a little information on examples, but not full details of a schema and input/output data, and not for every supported function. We could add tables for the fields in the schema, and for some example data.