doc: example not using cls or self.__class__

(sorry this is just a question...not a bug. didn't know where to go just for a question. maybe open a github discussion?)

in the [example](https://returns.readthedocs.io/en/latest/pages/interfaces.html#applicative),
```
...
.. @classmethod
... def from_value( # This method is required by Applicative
... cls,
... inner_value: _NewNumberType,
... ) -> 'Number[_NewNumberType]':
... return Number(inner_value)
```
why not use cls(inner_value)? also, i understand Applicative is a protocol/interface. but it's so generic. why not make it a mixin?