Docs for geojson

[steve-chavez]

Related to PostgREST/postgrest#1564.

We should have a new reference page dedicated to PostGIS. I was thinking this would be an "integration" but that would be more of a "how-to" and this is an actual reference - a technical description of the machinery and how to operate it.

[frafra]

Well, it would be already something to have in the documentation that PostgREST converts JSON input into (geometry, properties, type) when a GeoJSON feature is detected :)