Backport PR #12467: Move cell toolbar below search document widget (#12469)

Co-authored-by: Frédéric Collonval <fcollonval@users.noreply.github.com>

packages/cell-toolbar/style/base.css

@@ -15,7 +15,7 @@
   flex-direction: row;
   padding: 2px 0;
   min-height: 25px;
-  z-index: 10;
+  z-index: 6;
   position: absolute;
   top: 5px;
   right: 8px;