Add a section about MR validation against different Gitlab installation types to Code Review Guidelines

Proposal

Code Review Guidelines (https://docs.gitlab.com/ee/development/code_review.html) should cover testing the MR / validating the change against different types of installations, where we have: GDK/GCK, Omnibus, source installation, Cloud Native.

It would be really helpful to have a reminder to consider various installation types.
As well as any additional information that would help author and reviewer to be sure the change will work correctly under any GitLab installation type.

Relevant examples:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31056#note_195884812 - process name value for unicorn ($0) is different across GDK/Omnibus usage.

https://gitlab.com/gitlab-org/gitlab-ce/issues/65008#note_195178872 - a similar situation that led to a bug.