Combine all GitLab workflow documentation to CONTRIBUTING.md
In https://gitlab.com/gitlab-org/gitlab-ce/issues/24865#note_25139273, @rymai mentioned:
Hmm, yes indeed, there seem to be some duplication/overlap between:
CONTRIBUTING.md
Engineering Workflow (https://about.gitlab.com/handbook/engineering/workflow/)
Communication -> GitLab Workflow (https://about.gitlab.com/handbook/communication/#gitlab-workflow)
Should we just fold everything into CONTRIBUTING.md to reduce confusion and overlap?