docs: existing flux support (#1905)

Author: danielorihuela

docs/ac-existing-flux/how-it-works.md

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+# Support existing Flux
+
+Agent Control supports using an existing Flux installation. In other words, we can install Agent Control in a k8s cluster where Flux is already present. However, for the moment, we **DON’T HAVE** extensive and complete support for that. There are some limitations to that feature.
+
+## Requirements
+
+> [!WARNING]  
+> **Unsupported Configuration**\
+> Ensure the cluster complies with the following requirements. Configurations that deviate from these specifications are not validated or supported and may result in unexpected behavior or instability.
+
+* Flux version 2
+    * Helm Controller component
+        * HelmRelease CRD from helm.toolkit.fluxcd.io/v2
+    * Source Controller component
+        * HelmRepository CRD from source.toolkit.fluxcd.io/v1
+* ClusterRole for Flux with sufficient permissions
+* Flux is configured to watch resources in the namespace where Agent Control will be installed
+
+## How do we configure AC to work with an already existing Flux?
+
+Disable agent-control-cd. This is very straightforward.
+
+```yaml
+agentControlCd:
+  enabled: false
+```
+
+The final config would look something like the following:
+
+```yaml
+global:
+  cluster: "xxx"
+  licenseKey: "xxx"
+
+agentControlCd:
+  enabled: false
+
+agentControlDeployment:
+  chartValues:
+    subAgentsNamespace: "newrelic"
+    config:
+      fleet_control:
+        fleet_id: "xxx"
+    systemIdentity:
+      organizationId: "xxx"
+      parentIdentity:
+        clientId: "xxx"
+        clientSecret: "xxx"
+```
+
+## What's the minimum set of permissions required for the Cluster Role?
+
+This depends on the agents that we plan to install with Agent Control. The permissions is the sum of the permissions needed by:
+
+* `HelmController`
+* `SourceController`
+* Agent Control
+* Every agent we want to install
+
+Alternatively, we can use `cluster-admin`. This grants root privileges and it's [used by default in the Flux chart](https://github.com/fluxcd-community/helm-charts/tree/main/charts/flux2).
+
+## How does Flux watched namespaces influence Agent Control?
+
+Agent Control must be installed on a namespace watched by Flux. Otherwise, agents won't be installed. Now, we can find ourselves in two situations.
+
+First, Flux is configured to watch every namespace (`--watch-all-namespaces` is true). In that case, we can install Agent Control in any namespace and it will work out of the box.
+
+Second, Flux is configured to only watch the runtime namespace (`--watch-all-namespaces` is false). Then, we need to install Agent Control in the same namespace where Flux was installed.