Add validate yml to advanced troubleshooting

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -887,3 +887,14 @@ project = Project.find_by_full_path('<group/project>')
 
 Geo::RepositorySyncService.new(project).execute
 ```
+
+## Advanced Troubleshooting
+
+### Validate the `.gitlab-ci.yml`
+
+```ruby
+project = Project.find_by_full_path 'group/project'
+content = project.repository.gitlab_ci_yml_for(project.repository.root_ref_sha)
+Gitlab::Ci::YamlProcessor.validation_message(content,  user: User.first)
+```
+