[Documentation] Add doc/use_multiple_volumes_how_to.md (#612)

Signed-off-by: yandongxiao <[email protected]>

doc/use_multiple_volumes_how_to.md

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+# Deploy StarRocks with Multiple Volumes
+
+This document describes how to use multiple volumes to store StarRocks data.
+
+# Deploy StarRocks with Multiple Volumes By Helm Chart
+
+Based on the `storageSpec` field
+in [values.yaml](https://github.com/StarRocks/starrocks-kubernetes-operator/blob/main/helm-charts/charts/kube-starrocks/values.yaml),
+we will give an example of how to use multiple volumes to store StarRocks data.
+
+```yaml
+operator:
+  starrocksOperator:
+    image:
+      repository: starrocks/operator
+      tag: v1.9.8
+    imagePullPolicy: IfNotPresent
+    resources:
+      requests:
+        cpu: 1m
+        memory: 20Mi
+starrocks:
+  starrocksBeSpec:
+    beEnvVars:
+    # add storage_root_path in StarRocks config
+    config: |
+      be_port = 9060
+      webserver_port = 8040
+      heartbeat_service_port = 9050
+      brpc_port = 8060
+      sys_log_level = INFO
+      default_rowset_type = beta
+      storage_root_path = /opt/starrocks/be/storage0;/opt/starrocks/be/storage1
+    image:
+      repository: starrocks/be-ubuntu
+      tag: 3.2.2
+    replicas: 1
+    resources:
+      limits:
+        cpu: 8
+        memory: 8Gi
+      requests:
+        cpu: 1m
+        memory: 10Mi
+    storageSpec:
+      logStorageSize: 1Gi
+      name: be-storage
+      storageCount: 2   # specify the number of volumes
+      storageSize: 10Gi # the size of each volume
+  starrocksCluster:
+    enabledBe: true
+    enabledCn: true
+  starrocksCnSpec:
+    cnEnvVars:
+    # add storage_root_path in StarRocks config
+    config: |
+      sys_log_level = INFO
+      # ports for admin, web, heartbeat service
+      thrift_port = 9060
+      webserver_port = 8040
+      heartbeat_service_port = 9050
+      brpc_port = 8060
+      storage_root_path = /opt/starrocks/cn/storage0;/opt/starrocks/cn/storage1;/opt/starrocks/cn/storage2
+    image:
+      repository: starrocks/cn-ubuntu
+      tag: 3.2.2
+    replicas: 1
+    resources:
+      limits:
+        cpu: 8
+        memory: 8Gi
+      requests:
+        cpu: 1m
+        memory: 10Mi
+    storageSpec:
+      logStorageSize: 1Gi
+      name: cn
+      storageCount: 3   # specify the number of volumes
+      storageSize: 10Gi # the size of each volume
+  starrocksFESpec:
+    image:
+      repository: starrocks/fe-ubuntu
+      tag: 3.2.2
+    resources:
+      limits:
+        cpu: 2
+        memory: 4Gi
+      requests:
+        cpu: 1m
+        memory: 20Mi
+    storageSpec:
+      logStorageSize: 1Gi
+      name: fe
+      storageSize: 10Gi
+```
+
+Note:
+
+1. add `storage_root_path` field in StarRocks config.
+2. use `storageCount` to specify the number of volumes.
+3. the `storage` directory still exists in the container, but will not be used to store data.