Add documentation for the packaging module.

This updates the user guide to refer to Packaging instead of Distutils.
Some files still require an update.
diff --git a/Doc/contents.rst b/Doc/contents.rst
index e938fcd..e9d1771 100644
--- a/Doc/contents.rst
+++ b/Doc/contents.rst
@@ -11,7 +11,7 @@
    library/index.rst
    extending/index.rst
    c-api/index.rst
-   distutils/index.rst
+   packaging/index.rst
    install/index.rst
    documenting/index.rst
    howto/index.rst