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.

Doc/library/python.rst

@@ -16,6 +16,7 @@ overview:
    builtins.rst
    __main__.rst
    warnings.rst
+   dataclasses.rst
    contextlib.rst
    abc.rst
    atexit.rst