* Update PP backbones‘ introduction

Update PP backbones‘ introduction

* Update sample image link

Update sample image link

* delete paddlecv whl package

delete paddlecv whl package

* Add model icons of OCR and Class

Add icons of OCR and Class

* test=documents_fix,test=release/2.3

* test=documents_fix,test=release/2.3

* add pplcnet

* add pplcnetv2

* add pphgnet

* fix

* update

* support gradio for PPLCNet PPLCNetV2 PPHGNet

* fix

* enrich contents

* Update benchmark_cn.md

* Update download_cn.md

* Update info.yaml

* Update info.yaml

* add example

* Add modelcenter en

* Add PP-HGNet en

* Add PP-LCNet and PP-LCNetV2 en

* Add modelcenter English

* rename v2 to V2

* fix

* Update info.yaml
Co-authored-by: NliuTINA0907 <65896652+liuTINA0907@users.noreply.github.com>
Co-authored-by: zhangyubo0722 <zhangyubo0722@163.com>

* test = add the PP-YOLOv2 Demo to ModerCenter

* test = update the PP-YOLOv2 Demo to ModerCenter

* test = update the PP-YOLOv2 Demo to ModerCenter

* test = add the folders for related models
Co-authored-by: NYang <yangyehua@baidu.com>