[FEAT]: add docstring for all functions as comments and as in doc

[lwaekfjlk]

Description

need to use sphinx-apidoc to achieve auto link

Additional Information

No response

[Monstertail]

[Refs] I recommend 4, which is quite detailed. 3 could be supplementary. @lwaekfjlk

1. How to apply sphinx to generate API doc
2. (Chinese)使用sphinx生成项目文档
3. Official sphinx APIs
4. A Brief Tutorial On Making Beauty Documents! with Another generation example and problem solving

[Monstertail]

[break down the issue to steps]

1. add Sphinx to project dependencies--see dependencies for Sphinx here;
2. modify the style of code comments to styles that Sphinx can find and extract. See styles here
3. Update docs when we change the APIs. Can the updating be automatic or we need to do it every time we change APIs? @lwaekfjlk

Two questions: @lwaekfjlk (1) Any suggestions for the plan? (2) Shall we start the API doc right now or after the decison of the most of API interface?

[lwaekfjlk]

@Kunlun-Zhu