Add release notes page for version 27.0.0

Tempest 27.0.0 has been released [1].
This commit adds a new page for 27.0.0 release notes.

[1] https://review.opendev.org/c/openstack/releases/+/783349

Change-Id: I7f055c9724ce39145125664db8cc8e1fc168314f
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 9b1719f..ed0a09f 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   v27.0.0
    v26.1.0
    v26.0.0
    v24.0.0