Add testing of links in docs

Depends on !164 (merged).

.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
 
 
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