bpo-32436: Add docs for contextvars (GH-5685)

(cherry picked from commit bd093355a6aaf2f4ca3ed153e195da57870a55eb)

Co-authored-by: Yury Selivanov <yury@magic.io>
diff --git a/Doc/library/index.rst b/Doc/library/index.rst
index a925e10..da6a460 100644
--- a/Doc/library/index.rst
+++ b/Doc/library/index.rst
@@ -55,6 +55,7 @@
    crypto.rst
    allos.rst
    concurrency.rst
+   contextvars.rst
    ipc.rst
    netdata.rst
    markup.rst