Headers: Fix duplicate oldframe header links

Author: nginx-jack

exampleSite/content/test-product/everything.md

@@ -69,4 +69,9 @@ This won't render anything.
 <details open>
     <summary>Learn how to pin NGINX Plus to a specific version</summary>
     And this is the content on how to do so.
-</details>
+</details>
+
+## [Heading with link](https://nginx.org/)
+
+This is some text, under a heading with a link.
+There was a bug, where if a heading had a link in it, it would get duplicated in old frame.

layouts/_default/_markup/render-heading.html

@@ -7,7 +7,9 @@
     </a>
 </div> 
 
-<a class="headerlink float-right" data-mf="true" style="display: none;" href="#{{ .Anchor | safeURL }}" title="{{ .Text | safeHTML }}">
+<div data-mf="true">
+<a class="headerlink float-right" href="#{{ .Anchor | safeURL }}" title="{{ .Text | safeHTML }}">
     {{ .Text | safeHTML }}
 </a>
+</div>
 </h{{ .Level }}>