Add document for checking MR inclusion check on auto-deploy packages

This change adds documentation for the new package_check feature that helps quickly verify if specific MRs are included in deployed auto_deploy packages.

The feature is particularly useful during incidents when teams need to rapidly determine whether a bug fix or problematic change has been deployed to production. It works through Slack commands that can check either a specific package version or scan through recent packages, and can detect changes that were either directly included or cherry-picked for urgent fixes.

This tool eliminates the need for manual verification when investigating issues, especially when dealing with packages containing hundreds of code changes.

Related issue: gitlab-com/gl-infra/delivery#21583