Docs: please translate the ‘prob.rst’ (representation of probability) from English to Chinese

[LIUZhaohan0]

• You can find the doc under path: https://github.com/DataCanvasIO/YLearn/blob/main/docs/zh_CN/source/sub/causal_model/prob.rst
• Please firstly read the target doc and related docs if needed to fully understand the content
• Please translate the target English doc to Chinese and edit directly on the doc itself.