De-duplicate and reorganize setuptools documentation

[xmunoz]

The documentation for setuptools has grown messily over time and is difficult to browse and
navigate. Also, the legacy documentation for distutils and the current setuptools docs heavily overlap in content. These references thus trip up even experienced developers who want to understand these fundamental utilities.

We need funding for several weeks of technical writer work and developer review to:

• de-duplicate the distutils and setuptools documentation, making the latter independent of the former
• re-organize the setuptools documentation