Improve existing (minimal) Documentation

[SwapneelM]

Documentation is available, but we need a complete set of docs relating to what each function (or at least submodule i.e. DeepJetCore.preprocessing, DeepJetCore.evaluation, etc.) is meant for and the parameters that go in and out of each function within these submodules.

Bare minimum: Explain how DeepJetCore is structured and the idea behind each submodule if not docstrings for each function.

See also the process of generating documentation (and a PDF file containing all docs) based on the ReadTheDocs theme.

You want to start off by building your own version of the docs using the make html or other commands available in this Makefile.

Note that this is on the python-package branch, not master.