bpo-32216: Add documentation for dataclasses (GH-6886)


This is an initial version that likely requires much polishing. I'm adding it lay out the structure and so we have something to start working from.
(cherry picked from commit 98d50cb8f57eb227c373cb94b8680b12ec8aade5)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>
diff --git a/Doc/library/python.rst b/Doc/library/python.rst
index 440dc66..f39613f 100644
--- a/Doc/library/python.rst
+++ b/Doc/library/python.rst
@@ -16,6 +16,7 @@
    builtins.rst
    __main__.rst
    warnings.rst
+   dataclasses.rst
    contextlib.rst
    abc.rst
    atexit.rst