Add airgap topic page

Relates to https://gitlab.com/gitlab-org/gitlab/issues/11520

doc/topics/airgap/index.md

+# Air-gapped GitLab
+
+Computers in an air-gapped network are isolated from the public internet as a security measure.
+This page lists all the information available for running GitLab in an air-gapped environment.
+
+## Features
+
+Follow these best practices to use GitLab's features in an offline environment:
+
+- [Operating the GitLab Secure scanners in an offline environment](../../user/application_security/offline_deployments/index.md).