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

mkdocs.yml

@@ -13,7 +13,7 @@
 #   * fonts are self-hosted (publishing/fonts/) and OFL-licensed (Figtree).
 #   * no CDN / third-party host appears anywhere in the output.
 
-site_name: Inference Cooperative
+site_name: Inference Cooperative docs
 site_description: >-
   Member-readable documentation for the Inference Cooperative: how to join and
   use the service, the model catalogue, API access, governance and policies.