Update docs for generic and *_demo datasets w/ Klaus' comment

[maurolepore]

Here @2diiKlaus explains how the structure changes along the analysis process for market, portfolio, and loanbook datasets.

I see an opportunity to capture that knowledge in the documentation of generic datasets and *_demo datasets.

In writing these documents, it would be nice to engage @2diiKlaus, @vintented, and @jdhoffa.