Developer guidelines for correlation IDs

What does this MR do?

This adds developer guidelines for how to work with correlation IDs. This is in response to an incident (gitlab-com/gl-infra/production#1710 (comment 295321877)).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content