Add "Arbiter" document to Architecture

docs/architecture/arbiter.md

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+---
+title: Arbiter
+---
+
+# Arbiter 
+
+## About This Project
+
+The external-arbiter-operator (Arbiter) works with Rook-provisioned Ceph
+clusters to deploy external arbiters (monitors) that are not managed by Rook
+but that participate in consensus.
+
+The operator also monitors the remote cluster to verify its availability and
+ensure that the tenant has sufficient permissions to handle the deployment of
+Arbiter.
+
+## Requirements and Setup
+
+### Required Tools
+
+The following tools are required on your development machine:
+
+- `sed`
+- `openssl`
+- `make`
+- `git`
+- `golang`
+- `lima` (or another method to provision Kubernetes locally, such as Minikube)
+- `kubectl`
+- `docker` (or any compatible container engine, such as Podman)
+- `helm`
+
+The remaining dependencies are provisioned via Go tools, including the
+Kubebuilder toolset.
+
+## Quick Start
+
+What follows is a quick walkthrough on how to prepare the environment, run the
+operator locally, and deploy an external monitor.
+
+### Clone and Setup
+
+```bash
+# Clone the Rook repository: https://github.com/rook/rook 
+
+#Run `make deps`:
+make deps
+
+# Create OSD for Ceph
+limactl disk create osd --size=8G
+
+# Create VM instance
+limactl create --name=k8s ./contrib/vm.yaml
+
+# Start VM
+limactl start k8s
+
+# Use kubeconfig provided by VM
+export KUBECONFIG="${HOME}/.lima/k8s/copied-from-guest/kubeconfig.yaml"
+```
+
+### Install Prerequisites
+
+```bash
+# Install cert-manager
+kubectl apply -f https://github.com/cert-manager/cert-manager/releases/download/v1.19.2/cert-manager.yaml
+
+# Install Rook operator
+kubectl apply -f ./rook/deploy/examples/crds.yaml
+kubectl apply -f ./rook/deploy/examples/common.yaml
+kubectl apply -f ./rook/deploy/examples/operator.yaml
+kubectl apply -f ./rook/deploy/examples/csi-operator.yaml
+
+# Create Ceph cluster
+kubectl apply -f ./rook/deploy/examples/cluster-test.yaml
+
+# (Optional) Install Ceph toolbox
+kubectl apply -f ./rook/deploy/examples/toolbox.yaml
+```
+
+### Build and Install Operator
+
+```bash
+# Build image
+limactl shell k8s sudo nerdctl --namespace k8s.io build \
+  -t localhost:5000/cobaltcore-dev/external-arbiter-operator:latest \
+  -f ./Dockerfile .
+
+# Dry run operator install via Helm
+helm install --dry-run --create-namespace --namespace arbiter-operator \
+  --values ./contrib/charts/external-arbiter-operator/local.yaml \
+  arbiter-operator ./contrib/charts/external-arbiter-operator
+
+# Install operator via Helm chart
+helm install --create-namespace --namespace arbiter-operator \
+  --values ./contrib/charts/external-arbiter-operator/local.yaml \
+  arbiter-operator ./contrib/charts/external-arbiter-operator
+```
+
+### Configure and Deploy Arbiter
+
+```bash
+# Create namespace, user, role, rolebinding, kubeconfig and secret for arbiter
+./hack/configure-k8s-user.sh
+
+# Create secret with remote cluster access configuration
+kubectl apply -f ./contrib/k8s/examples/secret.yaml -n arbiter-operator
+
+# Create remote cluster resource
+kubectl apply -f ./contrib/k8s/examples/remote-cluster.yaml -n arbiter-operator
+
+# Create remote arbiter resource
+kubectl apply -f ./contrib/k8s/examples/remote-arbiter.yaml -n arbiter-operator
+
+# Watch until Arbiter is ready
+kubectl get remotearbiter -n arbiter-operator -w
+
+# Check that Arbiter has joined quorum
+kubectl exec deployment/rook-ceph-tools -n rook-ceph -it -- ceph mon dump
+```
+
+### Cleanup
+
+```bash
+# Remove Helm chart
+helm uninstall --namespace arbiter-operator arbiter-operator
+
+# Stop VM
+limactl stop k8s
+
+# Delete VM
+limactl delete k8s
+```
+
+## Make Goals
+
+Useful make commands for development:
+
+```bash
+# Build binary
+make
+
+# Prettify project, run linters, etc.
+make pretty
+
+# Run tests
+make test
+
+# Regenerate Kubernetes resources
+make gen
+
+# Copy CRD definitions to Helm chart
+make helm
+```
+
+## Configuration
+
+### Deployment Configuration
+
+Deployment manifests are managed by Helm. The `values.yaml` file lists all
+available configuration options.
+
+### Resource Configuration
+
+The following example resources are provided:
+
+- `secret.yaml` - Kubeconfig secret for arbiter installation
+- `remote-cluster.yaml` - RemoteCluster resource definition
+- `remote-arbiter.yaml` - RemoteArbiter resource definition
+
+## How to Run
+
+### Prerequisites
+
+Before running the operator, ensure the following conditions are met:
+
+1. A Ceph cluster operated by Rook is already up and running on the source
+   Kubernetes cluster
+1. Resources (pods, services) from the target (arbiter) cluster are reachable
+   from the source (operator/Rook) cluster and vice versa
+
+### Deployment Steps
+
+1. Create a user on the target cluster.
+1. Create the target namespace on the target cluster.
+1. Grant the user permissions to manage deployments, secrets, configmaps, their
+   statuses, and finalizers.
+1. Provision the target user kubeconfig on the source cluster via secret.
+1. Deploy the operator on the source cluster.
+1. Create a RemoteCluster resource on the source cluster, referencing the target
+   user kubeconfig secret.
+1. Create a RemoteArbiter resource on the source cluster, referencing the
+   RemoteCluster.
+1. Watch until resources are ready.
+1. Verify that the arbiter has joined the quorum by running `ceph mon dump`.
+
+## See Also
+[Arbiter Repository](https://github.com/cobaltcore-dev/external-arbiter-operator?tab=readme-ov-file)