docs: add link to open documentation issue on GitHub in footer

It uses templating to automatically get the repository URL from the
configuration file, and then appends the rest of the link to it.

Author: Tygo-van-den-Hurk

docs/overrides/templates/footer.html

@@ -3,6 +3,10 @@
     <div class="container-wrapper px-4 xl:px-6">
         <div class="flex h-(--footer-height) items-center justify-between">
             <div class="text-muted-foreground w-full text-center text-xs leading-loose sm:text-sm">
+                {% set repo_url = config.repo_url.rstrip('/') %}
+                These docs are <a href="{{ repo_url }}">open source</a>. See room for
+                improvement? <a href="{{ repo_url }}/issues/new?template=03-docs.yaml">Open an issue</a>,
+                or even better, <a href="{{ repo_url }}/compare">open a pull request</a>.
             </div>
         </div>
     </div>