docs: add missed doc strings

[Nihisil]

#121

I did final checks and noticed that https://mahjongrepository.github.io/mahjong/modules/hand_calculating/yaku.html# is not being rendered correctly (there is no yaku/__init__.py info here). I think it is because yaku.py didn't have module level doc.

[Nihisil]

@Apricot-S it is still not rendered 🫠 https://mahjongrepository.github.io/mahjong/modules/hand_calculating/yaku.html

but I guess it is minor issue and can be fixed later

[Apricot-S]

@Nihisil
It looks normal on my environment, but how about yours?

[Apricot-S]

The yaku/__init__.py file does not exist, and the Yaku.__init__() method is not defined, so it will not be rendered.

Where are you referring to?

[Nihisil]

@Apricot-S thanks for checking, all good now. I just checked wrong thing