Move documentation from code to documentation module

[Melevir]

Sphinx docs are great, but there are to many of them. I mean, look: 10 lines of code and 20 lines of docs - ordinary case in the project.
This dramatically reduces maintainability - to many scrolling per action rate.
Good idea is to keep small doc strings where it's required and move other info to docs/ folder.
Doctests are not a problem also since we have unit tests now.