Convert docstrings to numpydoc

[anmyachev]

Is your feature request related to a problem? Please describe.
Since in the near future a large number of new people will begin to work on the project, it is necessary to agree on a standard for docstrings. And introduce some kind of automatic checks.

For my part, I would like to suggest nympydoc package that we can use in github actions CI.

[devin-petersohn]

So far, the approach has been:

def some_method(self, arg1, arg2):
	"""The one-line description of the method.

	Args:
		arg1: Description
		arg2: Description

	Returns:
		What this returns (if anything)
	"""
	# Begin implementation
	return something

I am open to new suggestions as long as we are standardized. numpydoc seems reasonable, interested to hear other opinions.

[aregm]

Looks like numpydoc has a standard extension in sphinx, which will make the cross-reference easier.

[devin-petersohn]

numpydoc is a good testable standard. Let's go with that. We can install pydocstyle and test from bash with

pydocstyle --convention=numpy modin/pandas