Merge branch 'john_long-add-validate-yml-to-advanced-troubleshooting' into 'master'

Add validate yml to advanced troubleshooting

Closes gitlab-com/support/docs#66

See merge request gitlab-org/gitlab!30074

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -603,6 +603,14 @@ m = project.merge_requests.find_by(iid: )
 m.project.try(:ci_service)
 ```
 
+### 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)
+```
+
 ### Disable AutoDevOps on Existing Projects
 
 ```ruby