Add securing page to clusters documentation

doc/user/project/clusters/securing.md

+---
+stage: Defend
+group: container_security
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Securing your deployed applications
+
+GitLab makes it easy to secure applications deployed in [connected Kubernetes clusters](index.md).
+You can benefit from the protection of a [Web Application Firewall](../../../topics/web_application_firewall/quick_start_guide.md),
+[Network Policies](../../../topics/autodevops/stages.md#network-policy),
+or even [Container Host Security](../../clusters/applications.md#install-falco-using-gitlab-cicd).
+
+This page contains full end-to-end steps and instructions to connect your cluster to GitLab and
+install these features, whether or not your applications are deployed through GitLab CI/CD. If you
+use [Auto DevOps](../../../topics/autodevops/index.md)
+to build and deploy your application with GitLab, see the documentation for the respective
+[GitLab Managed Applications](../../clusters/applications.md)
+above.
+
+## Overview
+
+At a high level, the required steps include the following:
+
+- Connect the cluster to GitLab.
+- Set up one or more runners.
+- Set up a cluster management project.
+- Install a Web Application Firewall, Network Policies, and/or Container Host
+  Security.
+- Install Prometheus to get statistics and metrics in the
+  [threat monitoring](../../application_security/threat_monitoring/)
+  dashboard.
+
+### Requirements
+
+Minimum requirements (depending on the GitLab Manage Application you want to install):
+
+- Your cluster is connected to GitLab (ModSecurity, Cilium, and Falco).
+- At least one GitLab Runner is installed (Cilium and Falco only).
+
+### Understanding how GitLab Managed Apps are installed
+
+You install GitLab Managed Apps from the GitLab web interface with a one-click setup process. GitLab
+uses Sidekiq (a background processing service) to facilitate this.
+
+```mermaid
+  sequenceDiagram
+    autonumber
+    GitLab->>+Sidekiq: Install a GitLab Managed App
+    Sidekiq->>+Kubernetes: Helm install
+    Kubernetes-->>-Sidekiq: Installation complete
+    Sidekiq-->>-GitLab: Refresh UI
+```
+
+NOTE: **Note:**
+This diagram uses the term _Kubernetes_ for simplicity. In practice, Sidekiq connects to a Helm
+Tiller daemon running in a pod in the cluster.
+
+Although this installation method is easier because it's a point-and-click action in the user
+interface, it's inflexible and hard to debug. When something goes wrong, you can't see the
+deployment logs. The Web Application Firewall feature uses this installation method.
+
+However, the next generation of GitLab Managed Apps V2 ([CI/CD-based GitLab Managed Apps](https://gitlab.com/groups/gitlab-org/-/epics/2103))
+don't use Sidekiq to deploy. All the applications are deployed using a GitLab CI/CD pipeline and
+therefore GitLab Runners.
+
+```mermaid
+sequenceDiagram
+  autonumber
+  GitLab->>+GitLab: Trigger pipeline
+  GitLab->>+Runner: Run deployment job
+  Runner->>+Kubernetes: Helm install
+  Kubernetes-->>-Runner: Installation is complete
+  Runner-->>-GitLab: Report job status and update pipeline
+```
+
+Debugging is easier because you have access to the raw logs of these jobs (the Helm Tiller output is
+available as an artifact in case of failure) and the flexibility is much better. Since these
+deployments are only triggered when a pipeline is running (most likely when there's a new commit in
+the cluster management repository), every action has a paper trail and follows the classic merge
+request workflow (approvals, merge, deploy). The Network Policy (Cilium) Managed App and Container
+Host Security (Falco) are deployed with this model.
+
+## Connect the cluster to GitLab
+
+To deploy GitLab Managed Apps to your cluster, you must first
+[add your cluster](add_remove_clusters.md)
+to GitLab. Then [install](../../clusters/applications.md#installing-applications)
+the Web Application Firewall from the project or group Kubernetes page.
+
+Note that your project doesn't have to be hosted or deployed through GitLab. You can manage a
+cluster independent of the applications that use the cluster.
+
+## Set up a GitLab Runner
+
+To install CI/CD-based GitLab Managed Apps, a pipeline using a GitLab Runner must be running in
+GitLab. You can [install a GitLab Runner](../../clusters/applications.md#gitlab-runner)
+in the Kubernetes cluster added in the previous step, or use one of the shared runners provided by
+GitLab if you're using GitLab.com.
+
+With your cluster connected to GitLab and a GitLab Runner in place, you can proceed to the next
+steps and start installing the Cilium and Falco GitLab Managed Apps to secure your applications
+hosted on this cluster.
+
+## Create a Cluster Management Project
+
+A [Cluster Management Project](../../clusters/management_project.md)
+is a GitLab project that contains a `.gitlab-ci.yml` file to deploy GitLab Managed Apps to your
+cluster. This project runs the required charts with the Kubernetes
+[`cluster-admin`](https://kubernetes.io/docs/reference/access-authn-authz/rbac/#user-facing-roles)
+privileges.
+
+The creation of this project starts like any other GitLab project. Use an empty
+project and add a `gitlab-ci.yml` file at the root, containing this template:
+
+```yaml
+include:
+  - template: Managed-Cluster-Applications.gitlab-ci.yml
+```
+
+To make this project a Cluster Management Project, follow these
+[instructions](../../clusters/management_project.md#selecting-a-cluster-management-project).
+This project can be designated as such even if your application isn't hosted on GitLab. In this
+case, create a new empty project where you can select your newly created Cluster Management Project.
+
+## Install GitLab Container Network Policy
+
+GitLab Container Network Policy is based on [Cilium](https://cilium.io/). To
+install the Cilium GitLab Managed App, add a
+`.gitlab/managed-apps/config.yaml` file to your Cluster Management project:
+
+```yaml
+# possible values are gke, eks or you can leave it blank
+clusterType: gke
+
+cilium:
+  installed: true
+```
+
+Your application doesn't have to be managed or deployed by GitLab to leverage this feature.
+[Read more](../../clusters/applications.md#install-cilium-using-gitlab-cicd)
+about configuring Container Network Policy.
+
+## Install GitLab Container Host Security
+
+Similarly, you can install Container Host Security, based on
+[Falco](https://falco.org/), in your `.gitlab/managed-apps/config.yaml`:
+
+```yaml
+falco:
+  installed: true
+```
+
+[Read more] about configuring Container Host Security.