html: Wrap headline+content & headline content in containers

Org mode does it.

org/testdata/footnotes_in_headline.html

@@ -4,9 +4,11 @@
 </li>
 </ul>
 </nav>
+<div id="outline-container-headline-1" class="outline-2">
 <h2 id="headline-1">
 Title <sup class="footnote-reference"><a id="footnote-reference-1" href="#footnote-1">1</a></sup>
 </h2>
+</div>
 <div class="footnotes">
 <hr class="footnotes-separatator">
 <div class="footnote-definitions">