Merge branch 'add-docs-lint-back' into 'master'

Docs: Enable gitlab-docs lint

Closes gitlab-com/gitlab-docs#305

See merge request gitlab-org/gitlab-ce!23748

.gitlab-ci.yml

@@ -554,8 +554,7 @@ docs lint:
     # Build HTML from Markdown
     - bundle exec nanoc
     # Check the internal links
-    # Disabled until https://gitlab.com/gitlab-com/gitlab-docs/issues/305 is resolved
-    # - bundle exec nanoc check internal_links
+    - bundle exec nanoc check internal_links
 
 downtime_check:
   <<: *rake-exec