First, I think that it's good and useful to have full, well-maintained examples of how to build extension modules in the docs.

Second, until distutils is removed, I think we should keep the documentation for how to use it. Due to this, I don't think adding the "setuptools disclaimer" would be appropriate, since we shouldn't remove the section once the setuptools docs are updated.

It seems to me that we should:

1. Add a prominent note about distutils being deprecated and what that means: can one still use it, should one still use it, when is it planned to be removed, etc.

2. Add mention that 3rd-party tools are now the recommended way to build extensions. Include links to the sections in these libraries' docs explaining how to build extensions.