docs site: label the site as 'docs' and link the co-op homepage

Make clear this is the documentation, not the co-op homepage:
- site_name -> 'Inference Cooperative docs' (drives the <title> and the
  header brand on every page)
- homepage h1 -> 'Inference Cooperative docs'
- footer gains a 'Co-op homepage' link to https://inference.coop/, added
  in the existing copyright.html override alongside the Gitea source link

overrides/partials/copyright.html

@@ -4,6 +4,10 @@
       {{ config.copyright }}
     </div>
   {% endif %}
+  <div class="md-copyright__highlight">
+    Co-op homepage:
+    <a href="https://inference.coop/" rel="noopener">inference.coop</a>
+  </div>
   <div class="md-copyright__highlight">
     Documentation source:
     <a href="https://git.inference.coop/co-op/docs" target="_blank" rel="noopener">co-op/docs on Gitea</a>