Docs - initial commit#142

This layout a documentation system and the corresponding command - `nimble gen_book` - `nimble publish_book` to ease update. This targets writing guides, tutorials and how-to, i.e. from https://documentation.divio.com/  > - tutorials and how-to guides are both concerned with **describing practical steps** > - how-to guides and technical reference are both **what we need when we are at work, coding** > - reference guides and explanation are both concerned with **theoretical knowledge** > - tutorials and explanation are both **most useful when we are studying**, rather than actually working  Regarding API doc Ideally: - https://github.com/nim-lang/Nim/issues/14375 - https://github.com/nim-lang/Nim/issues/14376 would help generate html or json that can be post-processed and included into the book.