Add file starting point

Many commits squashed into one after I fought the danger-bot for more
time than I care to admin.

Add high-level steps of airgap

Add a link on Secure page & flesh out offline page more

Fix some markdown lint errors

Remove license scanning placeholder

Revert unintentional package change

Apply suggestion to
 doc/user/application_security/offline_deployments/index.md

Apply suggestion to
 doc/user/application_security/offline_deployments/index.md

Apply suggestion to
 doc/user/application_security/offline_deployments/index.md

Update text to mention package repos

Add SAST link

Address feedback given during the review

Add DAST link

Remove dependency scanning link until its merged

doc/user/application_security/index.md

@@ -240,6 +240,15 @@ An approval is optional when a license report:
 - Contains no software license violations.
 - Contains only new licenses that are `approved` or unknown.
 
+## Working in an offline environment
+
+It is possible to run most of the GitLab security scanners when not
+connected to the internet, in what is sometimes known as an offline,
+limited connectivity, Local Area Network (LAN), Intranet, or "air-gap"
+environment.
+
+Read how to [operate the Secure scanners in an offline environment](offline_deployments/index.md).
+
 ## Outdated security reports
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/4913) in GitLab 12.7.

doc/user/application_security/offline_deployments/index.md

+---
+type: reference, howto
+---
+
+# Offline deployments
+
+This document describes how to operate Secure scanners offline.
+
+## Overview
+
+It is possible to run most of the GitLab security scanners when not
+connected to the internet, in what is sometimes known as an offline,
+limited connectivity, Local Area Network (LAN), Intranet, or "air-gap"
+environment.
+
+In this situation, the GitLab instance can be one, or more, servers and services running in a network that can talk to one another, but have zero, or perhaps very restricted access to the internet. Assume anything within the GitLab instance and supporting infrastrusture (private maven repository for example) can be accessed via local network connection. Assume any files from the internet must come in via physical media (USB drive, hard drive).
+
+GitLab scanners generally will connect to the internet to download the
+latest sets of signatures, rules, and patches. A few extra steps are necessary
+to configure the tools to not do this and to still function properly.
+
+### Container registries and package repositories
+
+At a high-level, each of the security analyzers are delivered as Docker
+containers and reference various package repositories. When you run a job on
+an internet-connected GitLab installation, GitLab checks the GitLab.com-hosted
+container registry and package repositories to ensure that you have
+the latest versions.
+
+In an air-gapped environment, this must be disabled so that GitLab.com is not
+queried. Because the GitLab.com registry and repositories are not available,
+you must update each of the scanners to either reference a different,
+internally-hosted registry or provide access to the individual scanner images.
+
+You must also ensure that your app has access to common package repos
+that are not hosted on GitLab.com, such as npm, yarn, or rubygems. Packages
+from these repos can be obtained by temporarily connecting to a network or by
+mirroring the packages inside your own offline network.
+
+### Scanner signature and rule updates
+
+When connected to the internet, some scanners will reference public databases
+for the latest sets of signatures and rules to check against. Without connectivity,
+this is not possible. Depending on the scanner, you must therefore disable
+these automatic update checks and either use the databases that they came
+with or manually update those databases.
+
+## Specific scanner instructions
+
+Each individual scanner may be slightly different than the steps described
+above. You can find more info at each of the pages below:
+
+- [Container scanning offline directions](../container_scanning/index.md#running-container-scanning-in-an-offline-air-gapped-installation)
+- [SAST offline directions](../sast/index.md#gitlab-sast-in-an-offline-air-gapped-installation)
+- [DAST offline directions](../dast/index.md#running-dast-in-an-offline-air-gapped-installation)