* Add documentation to the top-level GeoParquet description and definition.
* Add the geometry_bbox definition to the json schema
* Add a few tests. Verify with `pytest test_json_schema.py`

Change the geometry_bbox to the broader "covering" section. Update
tests and examples.

Made some documentation updates:
* Parquet schema -> group
* Do not require zmin/zmax if geometries have 3 dimensions

Refactor the covering bbox to be an object with fields specifying
each bbox element. "Pin" them to have to end in ".xmin", ".xmax", etc.

Fixes failing test

Describe bounding box as the "coordinate range" of the geometry which is
language used by the GeoJSON spec

Update bbox so that each element is an array. For example:
["bbox", "xmin"] which represents a top-level bbox struct field
with an xmin member beneath it

Accidentally committed in last version for testing

* Mention that bounding boxes are useful for both row group
statistics and page indexes
* Require that bbox encoding columns be all of the same type (float or double)