Nbdev for Clay Documentation, Clay SDK and Clay notebooks all at once

I've started to work on documentation. I decided to start over using nbdev

It's a bit convoluted and I'm over my head coding here, but it seems highly attractive to do all this all at once without extra work:

Explore the model with notebooks anyone can run.
All functions and Classes created can be pip installed, making it a small SDK for Clay.
Notebooks help generate documentation website for all functions, with links to the code
Documentation and sample code tests run on CI.

@weiji14 your work is still in the history, and we can revert to that. I've migrated your text to this PR.

It's early, but I wanted to give a heads up in case others have experience on this.

GH Action is already rendering a draft site on https://clay-foundation.github.io/model/

I've also focused on the v0 release notes, but we still miss a lot of information. WIP.

Clay-foundation / model