Move JSON Schema definition to format-specs/

[kylebarron]

For #64. This is intended to make the JSON Schema definition a more "focal" part of the specification.

The symlink here gets resolved when building the Python package, so a wheel or sdist will still include a physical copy of the schema:

> python setup.py sdist > /dev/null && tar -tf dist/geoparquet_validator-0.0.1.tar.gz | grep schema.json
geoparquet_validator-0.0.1/geoparquet_validator/schema.json

[cholmes]

Note that once we do this then any PR that updates the text likely should also update the JSON file. I think this is a win, but we probably should note that somewhere, and be sure to catch it in reviews. Thinking about like #94 - wondering if it needs a JSON schema update?

[kylebarron]

Thinking about like #94 - wondering if it needs a JSON schema update?

Yes the json schema definition would need to be updated to allow null.