Ignore favicon in html check

Favicons seem to be required by just-the-docs
Corresponding issue is opened in pmarsceill/just-the-docs#738

.gitlab-ci.yml

@@ -51,7 +51,7 @@ test-docs-links:
     - gem install bundler
     - bundle config set --local path 'vendor'
     - bundle install
-  script: bundle exec htmlproofer --assume-extension --allow_hash_href ./_site
+  script: bundle exec htmlproofer --assume-extension --allow_hash_href --url-ignore "/favicon.ico" ./_site
 
 
 # Theodolite Helm Chart