Add a short table of contents to the setup.cfg spec.

I tried shortening the sidebar ToC with the tocdepth option instead, but it has
a bug which caused all headings with a level deeper than the tocdepth value to
all have the same section number, which was a usability regression rather than
in improvement.

Doc/packaging/setupcfg.rst

@@ -12,6 +12,10 @@ This document describes the :file:`setup.cfg`, an ini-style configuration file
 (compatible with :class:`configparser.RawConfigParser`) used by Packaging to
 replace the :file:`setup.py` file.
 
+.. contents::
+   :depth: 3
+   :local:
+
 
 Syntax
 ======