[DOCS] Write Usage document and examples about scikit-hts

[vtoliveira]

I think a more concrete example would be better for usage section, I really like how koaning writes his library examples in their docs:

https://scikit-lego.readthedocs.io/en/latest/naive-bayes.html

So we could shorten our example notebooks to see how data is usually structured, how we transform it and then do a simple fit. What do you think?