Add PaddleDetection docs & some model configs

* add cn doc for INSTALL/GETTING_START

* cn for GETTING_STARTED

* fix doc

* Fix some docs
* Unify COCO and VOC
* Change PASCAL to Pascal
* Unify dataset/coco

* improve quality of images

* update url for install doc

* Rename object_detection to PaddleDetection
* Small fix for doc

* Revise README.md

* Revise `GETTING_STARTED.md`

* Revise `INSTALL.md`

* Add `CONFIG.md`

* Fix RetinaNet backbones in README

* Fix cascade backbones in README

* Fix capitalization

* Replace demo images and update visualize funcall

* Revise `DATA.md`

* Fix link to `DATA.md`

* Fix capitalization

* Add GC in train.py by defalut and change data co dataset in configs
* Update docs/INSTALL.md
* Enable build_strategy.enable_inplace = True

* Add INSTALL.md.