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

See merge request !3225