The GRiD API v3 is documented in swagger.v3.yaml. (Note that the API spec contains several template variables, e.g., {{ URL_ROOT }}, that will cause tools such as the Swagger Editor to throw errors. These need to be customized depending on which instance of GRiD you are accessing.)
GRiD also provides a WFS endpoint for querying point cloud and raster data. This is documented in wfs.rst.