Rearrange minterpy source package

The purpose of this merge request is a rearrangement of the minterpy package with the following goals:

• more consistent build process in preparation for future deployment. This follows loosely the development guidelines from scikit-hep, which is a very modern approach to packaging of python programs in science.
• fixing agreements on development practices and deployment.
• developing a consistent system for documentation.

To accomplish these objectives, no source code in src/minterpy was changed, except docstrings/__doc__ hooks. See the updated version of README.md and CONTRIBUTING.md for explanations of the different parts of the rearranged root directory.