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

README.md

@@ -1,4 +1,4 @@
-# Inference Cooperative
+# Inference Cooperative docs
 
 **AI, governed together.**