[Dygraph] update readme and polish some docs

[Dygraph] add model list doc, update model link in paddeocr.py and update model size in quick start doc

[Dygraph] update link in doc

[Dygraph] change output path

Fix the errors about description of dict

[Dygraph] update quickstart doc, model link uses link as a placeholder

update reademe for dygraph

update doc of db and installtion

fix doc algorithm_overview ch&en

change gpus to selected_gpus

[Dygraph] add py inference doc