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.
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