Document database review process
This adds details specific to the database review process here: https://ab-database-reviews.about-src.gitlab.com/handbook/engineering/workflow/code-review/database.html
This goes in line with: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25867
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6069
Edited by Andreas Brandl