Update README.md

Update README.md

Update models.md

Update training.md and inference_ppocr.md

fix link bug

fix bug

Update doc 2.3

add icdar2015 download

update icdar2015 download

Divide PP-OCR model inference and other model inference

Divide PP-OCR model inference and other model inference

update docs and fix typos

Add PP-OCR inference doc and more explanation

add 2.1 models to paddleocr whl

update readme