Standardize the Python docstrings style in the codebase

Currently, the docstrings in the codebase is written with a mix style: NumPyDoc and reStructuredText style.

The reST style may (to be confirmed) be more comprehensive especially when they are rendered. However, the raw docstrings of the style are harder to read than the NumpyDoc style. This is a disadvantage for people who are directly working with and reading the codebase.

We've considered to standardize the docstrings using NumpyDoc. Note that NumpyDoc requires additional extension of Sphinx (already in-place in the current minterpy docs).