Document `CI_SHARED_ENVIRONMENT` and `CI_DISPOSABLE_ENVIRONMENT`

What does this MR do?

GitLab runner 10.1 (gitlab-runner!690 (merged)) introduces 2 new environment variables: CI_SHARED_ENVIRONMENT and CI_DISPOSABLE_ENVIRONMENT

Preview: http://preview-docs-ci-disposable-environment.178.62.207.141.xip.io/ce/ci/variables/README.html#predefined-variables-environment-variables

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.