Add release notes page for version 22.0.0

Tempest 22.0.0 has been released [1].
This commit adds a new page for 22.0.0 release notes.

[1] https://review.opendev.org/#/c/685406/

Change-Id: Ic9437f871fb94f84f106590145177d3c6eabb345
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 1d0d914..43c102c 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v22.0.0
    v21.0.0
    v20.0.0
    v19.0.0