doc(volumes): add docs for attaching additional volumes

example/test_solrcloud_additional_volume.yaml

@@ -0,0 +1,52 @@
+# Mounting Additional Volumes
+#
+# It is also possible to mount additional volumes to the SolrCloud (or its initContainers). Some example scenarios where you might use this would be:
+#   - Custom solr plugins that rely on certain data to be present on disk
+#   - Bootstrapping additional configuration for Solr via a ConfigMap
+#
+# Below is an example of the last scenario:
+
+apiVersion: solr.apache.org/v1beta1
+kind: SolrCloud
+metadata:
+  name: test
+  namespace: test
+spec:
+  replicas: 1
+  customSolrKubeOptions:
+    podOptions:
+      volumes:
+        - source: 
+            configMap:
+              name: configset
+              defaultMode: 0777
+          name: configset
+          defaultContainerMount:
+            mountPath: /configset
+            name: configset
+      sidecarContainers:
+        - name: config-loader
+          image: alpine/curl:latest
+          command:
+            - "sh"
+            - "-c"
+            - "ls -la /configset"
+          volumeMounts:
+            - name: configset
+              mountPath: /configset
+
+# Note the `source` spec may change based on the implementation. In this case, the `configMap` spec requires field `name`.
+# See volume spec in CRD: https://github.com/apache/solr-operator/blob/main/config/crd/bases/solr.apache.org_solrclouds.yaml#L6800
+
+---
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: configset
+  namespace: test
+data:
+  config.xml: |
+    <?xml version="1.0" encoding="UTF-8" ?>
+     <config>
+      ...
+     </config>