Merge branch 'docs-app-dev-platform' into 'master'

Add docs for Application Development Platform

See merge request gitlab-org/gitlab-ce!29765

doc/topics/application_development_platform/index.md

+# Application Development Platform
+
+The GitLab Application Development Platform refers to the set of GitLab features that can be used by operations teams to 
+provide a full development environment to internal software development teams.
+
+## Overview
+
+The GitLab Application Development Platform aims to reduce and even eliminate the time it takes for an Operations team
+to provide a full environment for software developers. It comprises the following high-level elements:
+
+1. Compute
+1. Build, test, and deploy a wide range of applications
+1. Security
+1. Observability
+
+We believe the use of these common building blocks equate to big gains for teams of all sizes, resulting from the adoption
+of newer, more efficient, more profitable, and less error-prone techniques for shipping software applications.
+
+### Compute
+
+Because at GitLab we are [cloud-native first](https://about.gitlab.com/handbook/product/#cloud-native-first) our
+Application Development Platform initially focuses on providing robust support for Kubernetes, with other platforms
+to follow. Teams can bring their own clusters and we will additionally make it easy to create new infrastructure
+with various cloud providers.
+
+### Build, test, deploy
+
+In order to provide modern DevOps workflows, our Application Development Platform will rely on
+[Auto DevOps](https://docs.gitlab.com/ee/topics/autodevops/) to provide those workflows. Auto DevOps works with 
+any Kubernetes cluster; you're not limited to running on GitLab's infrastructure. Additionally, Auto DevOps offers 
+an incremental consumption path. Because it is [composable](https://docs.gitlab.com/ee/topics/autodevops/#using-components-of-auto-devops),
+you can use as much or as little of the default pipeline as you'd like, and deeply customize without having to integrate a completely different platform.
+
+### Security
+
+The Application Development Platform helps you ensure that the applications you create are not affected by vulnerabilities 
+that may lead to security problems and unintended use. This can be achieved by making use of the embedded security features of Auto DevOps,
+which inform security teams and developers if there is something to consider changing in their apps 
+before it is too late to create a preventative fix. The following features are included:
+
+- [Auto SAST (Static Application Security Testing)](https://docs.gitlab.com/ee/topics/autodevops/#auto-sast-ultimate)
+- [Auto Dependency Scanning](https://docs.gitlab.com/ee/topics/autodevops/#auto-dependency-scanning-ultimate)
+- [Auto Container Scanning](https://docs.gitlab.com/ee/topics/autodevops/#auto-container-scanning-ultimate)
+- [Auto DAST (Dynamic Application Security Testing)](https://docs.gitlab.com/ee/topics/autodevops/#auto-dast-ultimate)
+
+### Observability
+
+Performance is a critical aspect of the user experience, and ensuring your application is responsive and available is everyone's
+responsibility. The Application Development Platform integrates key performance analytics and feedback 
+into GitLab, automatically. The following features are included:
+
+- [Auto Monitoring](https://docs.gitlab.com/ee/topics/autodevops/#auto-monitoring)
+- [In-app Kubernetes Pod Logs](https://docs.gitlab.com/ee/user/project/clusters/kubernetes_pod_logs.html)
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