Update master for stable/2026.1

Add file to the reno documentation build to show release notes for
stable/2026.1.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/2026.1.

Sem-Ver: feature
Change-Id: Ide064623268a8e4b9801f7755f6a0b2cf45fbced
Signed-off-by: OpenStack Release Bot <infra-root@openstack.org>
Generated-By: openstack/project-config:roles/copy-release-tools-scripts/files/release-tools/add_release_note_page.sh

releasenotes/source/2026.1.rst

@@ -0,0 +1,6 @@
+===========================
+2026.1 Series Release Notes
+===========================
+
+.. release-notes::
+   :branch: stable/2026.1

releasenotes/source/index.rst

@@ -6,6 +6,7 @@ Cinder Release Notes
    :maxdepth: 1
 
    unreleased
+   2026.1
    2025.2
    2025.1
    2024.2