Add chinese docs

[Blackwen]

I added Simplified Chinese documentation to the project. All Chinese documentation can be found in the docs_chs/ directory. I also added a button to link docs_chs/README.chs.md in the README file.

[Goooler]

@Blackwen 建议给所有中文和英文字符之间插入一个空格，让排版更好看

[kizitonwose]

Not sure about merging this, I feel like non-English docs would get outdated very quickly since I cannot update them. So library users could be looking at outdated docs without even realizing it. For example, with the recent documentation updates due to multiplatform changes, the docs in this MR are slightly outdated already. I think non-English users are probably better off translating the desired documentation markdown file with an online tool.

[Goooler]

That's the thing I worry about too.

[kizitonwose]

Closing this as the concern regarding being outdated is now fully manifested after I updated the docs for the newly added year-based calendar.