Improve structure of docs.

[GJBoth]

The deepmod class is our base and its combined together with all the other abstract classes. I'm not sure this is the most clear; should we move the abstract classes to each instance it belongs to? @GijsVermarien @remykusters @georgestod What do you think?

[remykusters]

Yes, I fully agree on that one, it was confusing to me as well.

[GijsVermarien]

Also agreed, maybe we can provide a documentation piece on the abstract base classes in the documentation separated from the code.

[georgestod]

I have not fully understood the structure of the docs yet. What would help maybe would be to inspire from a typical scikit-learn documentation: parameters, attributes + methods (and then parameters + returns).