Merge branch 'docs/exclude-qa-frontend' into 'master'

Exclude qa-frontend from running on docs-only branches

See merge request gitlab-org/gitlab-ce!23327

.gitlab-ci.yml

@@ -760,6 +760,7 @@ qa:selectors:
     - yarn install --frozen-lockfile --cache-folder .yarn-cache
     - date
     - yarn run webpack-prod
+  <<: *except-docs
 qa-frontend-node:6:
   <<: *qa-frontend-node