* add PrepareDataSet.md, test=document_fix

* update PrepareDataSet, test=document_fix

* update PrepareDataSet, test=document_fix

Repalce fruit dataset with roadsign dataset, and update QUICK_STARTED, use PP-YOLO as demo, test=document_fix. (#1407)

* update INSTALL

* fix typo

* add demo image

* add use_gpu annotation

* fix link

* update style

* update style

* add install paddle

* use python3

* update INSTALL_cn.md

* update INSTALL_cn.md

* update INSTALL_cn.md

* fix typo

add fleet training

* fix voc dataset and docs

* add custom datsset demo

Add image for visualdl board.

* fix some docs link error

* rename config_doc

* fix export_model doc link error

* sort out release/0.3 docs

* add new CHANGELOG

* add yolov4 in changelog

* Update docs
* Update README.md

* optimize python path

* Remove `pycocotools` from `requirements.txt`

git version is required for numpy > 1.18

* Remove `cython`, add pip install doc

* add config link in model zoo

* fix link error

* add anchor free

* refine transfer learn

* refine transfer learning doc

* fix fruit config on master branch

* refine doc

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs