docs: update userguide index

docs/userguide/index.txt

+==================================================
+Building and Distributing Packages with Setuptools
+==================================================
+
+``Setuptools`` is a collection of enhancements to the Python ``distutils``
+that allow developers to more easily build and
+distribute Python packages, especially ones that have dependencies on other
+packages.
+
+Packages built and distributed using ``setuptools`` look to the user like
+ordinary Python packages based on the ``distutils``.
+
+.. toctree::
+    :maxdepth: 1
+
+    Quickstart <quickstart>
+    keyword reference <keywords>
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