docs: add parallel storage node addition documentation

docs/maintenance-operations/parallel-node-addition.md

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+---
+title: "Parallel Storage Node Addition"
+description: "How the Simplyblock operator adds storage nodes in parallel while preserving FoundationDB fault tolerance with sequential adds for FDB-hosting workers."
+weight: 10760
+---
+
+When a `StorageNode` resource is created with multiple worker nodes, the operator can add non-FDB workers
+concurrently rather than sequentially. This significantly reduces cluster provisioning time for large deployments.
+
+## How It Works
+
+The operator classifies each worker node into one of two groups before starting the add process:
+
+- **Non-FDB workers** — workers that do not host any FoundationDB process pods. These are added in parallel up
+  to the configured `maxParallelNodeAdds` limit.
+- **FDB workers** — workers running pods labelled `foundationdb.org/fdb-cluster-name`. These are always added
+  one at a time, in sequence.
+
+The sequential constraint for FDB workers exists because the storage node add process triggers a worker reboot.
+Rebooting multiple FDB nodes simultaneously reduces the number of available FDB coordinators below the quorum
+threshold, which would cause cluster unavailability.
+
+## Configuration
+
+Parallelism for non-FDB workers is controlled by `StorageNode.spec.maxParallelNodeAdds`.
+
+| Value | Behaviour |
+|-------|-----------|
+| `1` (default) | All workers added one at a time — safe for all topologies |
+| `> 1` | Up to N non-FDB workers added concurrently per reconcile pass |
+
+```yaml title="Enable parallel node addition"
+apiVersion: storage.simplyblock.io/v1alpha1
+kind: StorageNode
+metadata:
+  name: simplyblock-node
+  namespace: simplyblock
+spec:
+  clusterName: simplyblock-cluster
+  maxParallelNodeAdds: 5   # add up to 5 non-FDB workers at a time
+  workerNodes:
+    - worker-1
+    - worker-2
+    - worker-3
+    - worker-4
+    - worker-5
+    - worker-6
+    - worker-7
+    - worker-8
+```
+
+!!! note
+    `maxParallelNodeAdds` applies only to non-FDB workers. Workers hosting FoundationDB processes are always
+    added sequentially, regardless of this value.
+
+## Pinning FDB to Dedicated Nodes
+
+For the parallelism to be most effective, run FoundationDB on a dedicated subset of storage nodes rather than
+spreading it across all workers. Label the FDB-dedicated nodes before installation:
+
+```bash
+kubectl label node worker-1 worker-2 worker-4 simplyblock.io/fdb-node=true
+```
+
+Then configure the `FoundationDBCluster` resource to use only those nodes via a `nodeSelector`:
+
+```yaml
+spec:
+  processes:
+    general:
+      podTemplate:
+        spec:
+          nodeSelector:
+            simplyblock.io/fdb-node: "true"
+```
+
+With this setup, only the three labelled nodes are treated as FDB workers. All remaining workers are added
+in parallel.
+
+## Validation
+
+The following example shows a cluster with 8 storage workers where 6 non-FDB workers started in parallel.
+All SPDK pods entered `ContainerCreating` within the same reconcile pass:
+
+```
+NAME                        NODE                    STATUS
+snode-spdk-pod-4420-cb5317  worker-7   ContainerCreating
+snode-spdk-pod-4421-cb5317  worker-6   ContainerCreating
+snode-spdk-pod-4422-cb5317  worker-3   ContainerCreating
+snode-spdk-pod-4423-cb5317  worker-5   ContainerCreating
+snode-spdk-pod-4424-cb5317  worker-1   ContainerCreating
+snode-spdk-pod-4425-cb5317  worker-8   ContainerCreating
+```
+
+The two FDB workers (nodes 2 and 4) were then added sequentially. All 8 nodes came online and the cluster
+reached `ACTIVE` status.