Fix title duplication

Sphinx4 produces a different structure, so the CSS hack that was posted
to avoid duplication of titles no longer works. Apply the same hack to
the new structure.

See also: https://review.opendev.org/798897

Change-Id: Ic7eb8b90e5d68f390ef9015fa41dddac943cba3a

doc/source/_themes/releases/static/css/combined.css

@@ -4647,6 +4647,8 @@ table.highlighttable {
 .docs-body .section h1 {
   display: none;
 }
+.docs-body section h1 {
+  display: none; }
 
 .docs-top-contents a[href="#"] {
   display: none;