docs: image (#311)

* Now using __future__ type hints

* fix: config -> _config

* moved __future__ import to start

* _config import

* cupy import

* removed ' cupy array' type hint

* import config

* spacing between :

* formatting docstring

* style in example

* main concepts -> key features

* Docs, union removed, generic types used

* fix unclosed bracket

* removed Union in docs

* import iterable

* typo fix

* Type hints in docs

Author: Pwhsky