Add review apps documentation

Review apps are mappings of branch-environment. Creating a new doc will result in having documentation in 3 places:

• ci/environments.md
• ci/yaml/README.md
• ci/review_apps/

Todo

• Refactor environments https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7107
  • Migrate most of yaml's information in environment.md
  • Add section of dynamic environments to environment.md
  • Better explain deployments with images etc.
• Add new review apps doc that will link to both environments.md and yaml reference https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6982

Examples

• The simplest example is using review apps with NGINX https://gitlab.com/gitlab-examples/review-apps-nginx. Add full explanation in a tutorial-like way for the users to set up in their server with the xip.io domain.