Add 26.1.0 releasenote page

Add a new releasenote page for the latest 26.1.0 release
so that the releasenotes written within that release are
grouped together and showed properly.

Change-Id: I14c6743ce0d14d9265be5ee12d33f4955d0d8ada
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 21f414e..9b1719f 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v26.1.0
    v26.0.0
    v24.0.0
    v23.0.0