Add releasenotes page for version 40.0.0

Tempest 40.0.0 has been released [1].
This commit adds a new page for 40.0.0 release notes.

[1] https://review.opendev.org/c/openstack/releases/+/927833

Change-Id: I246e88b6dc261783fc397ff3bed9d39d3e7a2735
diff --git a/releasenotes/source/v40.0.0.rst b/releasenotes/source/v40.0.0.rst
new file mode 100644
index 0000000..995767b
--- /dev/null
+++ b/releasenotes/source/v40.0.0.rst
@@ -0,0 +1,6 @@
+=====================
+v40.0.0 Release Notes
+=====================
+
+.. release-notes:: 40.0.0 Release Notes
+   :version: 40.0.0