Add flake8-docstrings plugin to tox CI

[nstarman]

Description

It can be configured to check NumPy docstring style.

Additional context

See bottom of https://flake8.pycqa.org/en/latest/user/using-hooks.html.
It's a very official plugin.

[pllim]

Oh dear... I am sure we will get a lot of failures.

[nstarman]

Just think of the beautiful docs when we're done 🥺

[pllim]

By the time we are done, it is probably time to autodoc from type annotation or something... 🧓

[nstarman]

@pllim So true.
Anyway, ruff has docstrings stuff now that we should be using instead.