Docs: Add architecture of Auto DevOps

Pertaining to Auto Build and Auto Deploy.

Also how implicit yml works when a project has Auto DevOps enabled.

doc/development/README.md

@@ -68,6 +68,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Git LFS](lfs.md)
 - [Developing against interacting components or features](interacting_components.md)
 - [File uploads](uploads.md)
+- [Auto DevOps development guide](auto_devops.md)
 
 ## Performance guides
 

doc/development/auto_devops.md

+# Auto DevOps development guide
+
+This document provides a development guide for contributors to
+[Auto DevOps](../topics/autodevops/index.md)
+
+## Development
+
+Auto DevOps builds on top of GitLab CI to create an automatic pipeline
+based on your project contents. When Auto DevOps is enabled for a
+project, the user does not need to explicitly include any pipeline configuration
+through a [`.gitlab-ci.yml` file](../ci/yaml/README.md).
+
+In the absence of a `.gitlab-ci.yml` file, the [Auto DevOps CI
+template](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml)
+is used implicitly to configure the pipeline for the project. This
+template is a top-level template that includes other sub-templates,
+which then defines jobs.
+
+Some jobs use images that are built from external projects:
+
+- [Auto Build](../topics/autodevops/index.md#auto-build) uses
+  [configuration](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Jobs/Build.gitlab-ci.yml)
+  in which the `build` job uses an image that is built using the
+  [`auto-build-image`](https://gitlab.com/gitlab-org/cluster-integration/auto-build-image)
+  project.
+- [Auto Deploy](../topics/autodevops/index.md#auto-deploy) uses
+  [configuration](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Jobs/Deploy.gitlab-ci.yml)
+  in which the jobs defined in this template use an image that is built using the
+  [`auto-deploy-image`](https://gitlab.com/gitlab-org/cluster-integration/auto-deploy-image)
+  project. By default, the Helm chart defined in
+  [`auto-deploy-app`](https://gitlab.com/gitlab-org/charts/auto-deploy-app)
+  is used to deploy.
+
+There are extra variables that get passed to the CI jobs when Auto
+DevOps is enabled that are not present in a normal CI job. These can be
+found in
+[`ProjectAutoDevops`](https://gitlab.com/gitlab-org/gitlab/blob/bf69484afa94e091c3e1383945f60dbe4e8681af/app/models/project_auto_devops.rb).
+
+## Development environment
+
+Configuring [GDK for Auto
+DevOps](https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/auto_devops.md).

doc/topics/autodevops/index.md

@@ -1258,4 +1258,4 @@ curl --data "value=true" --header "PRIVATE-TOKEN: personal_access_token" https:/
 
 ## Development guides
 
-Configuring [GDK for Auto DevOps](https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/auto_devops.md).
+[Development guide for Auto DevOps](../../development/auto_devops.md)