Add docs for the PostHog cluster application

doc/user/clusters/applications.md

@@ -615,6 +615,7 @@ Supported applications:
 - [Crossplane](#install-crossplane-using-gitlab-cicd)
 - [Fluentd](#install-fluentd-using-gitlab-cicd)
 - [Knative](#install-knative-using-gitlab-cicd)
+- [PostHog](#install-posthog-using-gitlab-cicd)
 
 ### Usage
 
@@ -779,6 +780,55 @@ postgresql:
   postgresqlPassword: example-postgresql-password
 ```
 
+### Install PostHog using GitLab CI/CD
+
+[PostHog](https://www.posthog.com) 🦔 is a developer-friendly, open-source product analytics platform.
+
+To install PostHog into the `gitlab-managed-apps` namespace of your cluster,
+define the `.gitlab/managed-apps/config.yaml` file with:
+
+```yaml
+posthog:
+  installed: true
+```
+
+You can customize the installation of PostHog by defining `.gitlab/managed-apps/posthog/values.yaml`
+in your cluster management project. Refer to the [Configuration section of the PostHog chart's readme](https://github.com/PostHog/charts/tree/master/charts/posthog)
+for the available configuration options.
+
+NOTE: **Note:**
+You must provide a PostgreSQL password in `postgresql.postgresqlPassword`
+or you will receive authentication errors.
+See the [PostgreSQL chart documentation](https://github.com/helm/charts/tree/master/stable/postgresql#upgrade) for more information.
+
+Redis pods are restarted between upgrades. To prevent downtime, provide a Redis
+password using the `redis.password` key. This prevents a new password from
+being generated on each restart.
+
+Here is an example configuration for PostHog:
+
+```yaml
+ingress:
+  enabled: true
+  hostname: "<posthog.example.com>"
+
+# This will be autogenerated if you skip it. Include if you have 2 or more web replicas
+posthogSecret: 'long-secret-key-used-to-sign-cookies'
+
+# Needs to be here between runs.
+# See https://github.com/helm/charts/tree/master/stable/postgresql#upgrade for more info
+postgresql:
+  postgresqlPassword: example-postgresql-password
+
+# Recommended to set this to a value to redis prevent downtime between upgrades
+redis:
+  password: example-redis-password
+```
+
+NOTE: **Note:**
+Support for the PostHog managed application is provided by the PostHog team.
+If you run into issues, please [open a support ticket](https://github.com/PostHog/posthog/issues/new/choose) directly.
+
 ### Install GitLab Runner using GitLab CI/CD
 
 GitLab Runner is installed using GitLab CI/CD by defining configuration in