Add release notes page for version 31.1.0

Tempest 31.1.0 has been released [1].
This commit adds a new page for 31.1.0 release notes.

[1] https://review.opendev.org/c/openstack/releases/+/848090

Change-Id: I8752f3428e498c4b1fd2c12283bc0c97194423fe
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index e1e6597..9b5aad3 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v31.1.0
    v31.0.0
    v30.0.0
    v29.2.0