First draft on GitLab intro docs

[ci skip]

doc/intro/README.md

+## Organize
+
+> Create projects and groups
+
+- [Create a new project](../gitlab-basics/create-project.md)
+- [Create a new group](../gitlab-basics/create-group.md)
+
+## Prioritize
+
+> Create issues, labels, milestones, voting, and review issues
+
+- [Create a new issue](../gitlab-basics/create-issue.md)
+- [Assign labels to issues](../workflow/labels.md)
+- [Use milestones as an overview of your project's tracker](../workflow/milestones.md)
+- Use voting to express your like/dislike to the issue in demand
+
+## Collaborate
+
+> Create merge requests, diffs, commits, and review code
+
+- [Create a new merge request](../gitlab-basics/add-merge-request.md)
+- Review merge requests easily using inline comments
+
+## Deploy
+
+> Use built-in continuous integration in GitLab, or use your favorite CI tool.
+
+- [GitLab CI quick start guide](../ci/quick_start/README.md)
+
+## Install
+
+> Install and update your GitLab installation.
+
+- Install GitLab
+    - Omnibus
+    - [Source](../install/installation.md)
+- Upgrade GitLab
+- Learn about GitLab Omnibus architecture
+- GitLab Omnibus configuration options
+- Migrating from source installation to GitLab Omnibus