Restore heading self links (#2007)

It was a change I did not notice when upgrading Docsy from 0.8.0 to 0.11.0. Docsy changed the way heading self links are generated: they used to be rendered with JS and now they use a Hugo render hook.

This means two things:
- We need to enable them explicitly by overriding the `_default/_markup/render-heading.html` template.
- We need to add the self heading ourselves to headings that are not rendered by Hugo, i.e. HTML headings that we create ourselves.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

layouts/partials/openapi/render-operation.html

@@ -37,6 +37,7 @@
   <h1 id="{{ $anchor }}">
    <span class="http-api-method">{{ $method }}</span>
    <span class="endpoint{{ if $operation_data.deprecated }} deprecated-inline{{ end }}">{{ $endpoint }}</span>
+   {{ template "_default/_markup/td-heading-self-link.html" $anchor }}
   </h1>
 </summary>