Add cname: developer.gnustep.org. (#23)

Author: ivucica

.github/workflows/sphinx.yml

@@ -34,4 +34,5 @@ jobs:
       if: github.ref == 'refs/heads/master'
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
-        publish_dir: build/html
+        publish_dir: build/html
+        cname: developer.gnustep.org