Add release notes page for version 31.0.0

Tempest 31.0.0 has been released [1].
This commit adds a new page for 31.0.0 release notes.

[1] https://review.opendev.org/c/openstack/releases/+/843039

Change-Id: I290fd71dbe295087c0f3016efb541f2965f764d0
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 122f7c7..e1e6597 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v31.0.0
    v30.0.0
    v29.2.0
    v29.1.0
diff --git a/releasenotes/source/v31.0.0.rst b/releasenotes/source/v31.0.0.rst
new file mode 100644
index 0000000..8fb797c
--- /dev/null
+++ b/releasenotes/source/v31.0.0.rst
@@ -0,0 +1,5 @@
+=====================
+v31.0.0 Release Notes
+=====================
+.. release-notes:: 31.0.0 Release Notes
+   :version: 31.0.0