Add a new specifications section

[ncoghlan]

This adds a new specifications section, with the following goals:

• document how PyPA currently uses the PEP process
• provide a concise list of active specs and de facto standards
• lay the foundation for eventually changing the way we use the
  PEP process to have the text of the active specs live here under
  stable URLs, while the PEPs are used to argue for changes to
  the specifications (similar to the way python-dev uses PEPs to
  argue for changes to the CPython reference implementation)

[xavfernandez]

defined

[ncoghlan]

Rejecting this one, as after reflecting on it a bit more, I'd like to just have the process aspects here on pypa.io, and the specifications themselves on packaging.python.org.

More in the distutils-sig thread: https://mail.python.org/pipermail/distutils-sig/2016-January/028060.html