Docs/SDK/notebooks with nbdev [try 2]

[brunosan]

This take into account some of @weiji14 comments on #102 (which I abandoned to not deal with a polluted git history).

I will not spend much time on the architecture (nbdev vs other options). I do like the option to have all 3 assets at once (documentation htmls, SDK and notebooks) but I understand nbdev might be too rare on stale.

I'll focus meanwhile on the notebooks themselves and we can port the content to other tools.

In the end we need documentation asap.

[brunosan]

After thinking about it, I'll spin out this folder with documentation to a new repository. I think that our goal to create a good comprehensive documentation, templates, ... will bloat this repo with significantly functionally different content.

Spinning out on https://github.com/Clay-foundation/documentation/pull/1 I'll clean up the publish workflow on a PR next