GitLab intro docs

Related to https://gitlab.com/gitlab-org/marketing_monthly_release/issues/1

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Need refactor:

• Create a new project
• Create a new group
• Create a new issue
• Assign labels to issues
• Use milestones as an overview of your project's tracker
• Fork a project and contribute to it
• Create a new merge request
• Automatically close issues from merge requests (include GitLab.com pattern)
• GitLab CI quick start guide (make it easier to follow)

Moved to https://gitlab.com/gitlab-org/gitlab-ce/issues/8068