Release openstackdocstheme 2.3.1

The StarlingX docs site utilizes a theme variant within the
openstackdocstheme repo, but consumes packaged releases. A recent
change merged to fix an issue with double heading, and the StarlingX
community would appreciate a new openstackdocstheme release to
make use of it.

http://lists.openstack.org/pipermail/openstack-discuss/2021-September/024917.html

Change-Id: I56755457df76321a503ae5cc86690215ce4168a1

deliverables/_independent/openstackdocstheme.yaml

@@ -288,5 +288,9 @@ releases:
     projects:
       - repo: openstack/openstackdocstheme
         hash: 08461c5311aa692088a27eb40a87965fd8515aba
+  - version: 2.3.1
+    projects:
+      - repo: openstack/openstackdocstheme
+        hash: 4c6d65531645cd109062489c7bb06dd282d1c98f
 repository-settings:
   openstack/openstackdocstheme: {}