Add troubleshooting docs for downstream pipeline data integrity failures
What does this MR do and why?
This merge request adds documentation for troubleshooting a "data integrity failure" error that can occur in GitLab CI/CD jobs. The changes explain that this is a generic error message that can happen with any type of job, and provide guidance on checking PostgreSQL and Sidekiq logs to find the actual underlying problem. Common causes include database issues after upgrades, outdated background processes, or SQL problems from schema changes. The documentation also includes steps for further investigation using GitLab's Rails console if the error continues.
References
Edited by Max Fan