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?