bpo-34632: Add importlib.metadata (GH-12547)

Add importlib.metadata module as forward port of the standalone importlib_metadata.
diff --git a/Doc/library/modules.rst b/Doc/library/modules.rst
index 6b2a40a..565ce05 100644
--- a/Doc/library/modules.rst
+++ b/Doc/library/modules.rst
@@ -17,3 +17,4 @@
    modulefinder.rst
    runpy.rst
    importlib.rst
+   importlib.metadata.rst