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>

changelogs/internal/newsfragments/1996.clarification

@@ -1 +1 @@
-Upgrade docsy to 0.11.0 and hugo to 0.138.0.
+Upgrade docsy to 0.11.0 and hugo to 0.139.0.

changelogs/internal/newsfragments/2007.clarification

@@ -0,0 +1 @@
+Upgrade docsy to 0.11.0 and hugo to 0.139.0.