from quickstart.txt to functionalities.txt

doc overhaul step 2: break main doc into multiple sections

from pointing at setuptools.txt to userguide/index.txt

cut and paste from setuptools.txt

cut-and-paste from setuptools.txt

Did a cut and paste from setuptools.txt

Did a cut and paste from setuptoo.txt to userguide/quickstart.txt

doc overhaul step 1: simplify index and group deprecated files

Tweak note about setup.cfg

Emit deprecation warning when 2to3 is used.

The top index now points to three files: setuptools.txt (user guide),
development.txt (development guide) and a WIP index file that is to
summarized deprecated practice that remains relevant for backward
compatibility.

In the deprecation warning, acknowledge that it's only for projects that still require Python 2 support.

This note has gotten a bit out of date, since setup.py is no longer
required.

Filter lib2to3 (Pending)DeprecationWarning in testes

Added documentation for build_meta

lib2to3 is deprecated in Python 3.9: https://bugs.python.org/issue40360

Workarounds https://github.com/pypa/setuptools/issues/2081