Transition docs from setup.py to pyproject.toml

[BrennanBarker]

Is there any issue transitioning the "Setuptools Integration" doc section from referencing a notional setup.py to instead reference a pyproject.toml? I can submit a PR if there's no objection.

[swills1]

Just an FYI, @BrennanBarker is referring to this - https://github.com/pallets/click/blob/main/docs/setuptools.rst.

Which probably is antiquated at this point like they say.