add CVPR dataset to the doc
add CVPR dataset to the doc

Summary: Added LVIS dataset CVPR annotations description to the datasets doc.

Reviewed By: patricklabatut

Differential Revision: D28257722

fbshipit-source-id: d339f3c6ac2a624c9cce4fd01119412cd1e29461

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2