Add overview and use cases for MR-related docs

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Adds overview and use cases for:

• Merge requests
• Merge request approvals
• Multi-Project Pipeline Graphs (new doc)
• Code Quality reports

When approved:

• Create CE MR => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12875
• Update features.yml => https://gitlab.com/gitlab-com/www-gitlab-com/issues/1544

For https://gitlab.com/gitlab-org/gitlab-ee/issues/2703