Add volume expansion docs

Signed-off-by: Tapajit Chandra Paul <[email protected]>

docs/guides/druid/volume-expansion/_index.md

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+---
+title: Volume Expansion
+menu:
+  docs_{{ .version }}:
+    identifier: guides-druid-volume-expansion
+    name: Volume Expansion
+    parent: guides-druid
+    weight: 44
+menu_name: docs_{{ .version }}
+---

docs/guides/druid/volume-expansion/overview.md

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+---
+title: Druid Volume Expansion Overview
+menu:
+  docs_{{ .version }}:
+    identifier: guides-druid-volume-expansion-overview
+    name: Overview
+    parent: guides-druid-volume-expansion
+    weight: 10
+menu_name: docs_{{ .version }}
+section_menu_id: guides
+---
+
+> New to KubeDB? Please start [here](/docs/README.md).
+
+# Druid Volume Expansion
+
+This guide will give an overview on how KubeDB Ops-manager operator expand the volume of various component of `Druid` like:. (Combined and Topology).
+
+## Before You Begin
+
+- You should be familiar with the following `KubeDB` concepts:
+    - [Druid](/docs/guides/druid/concepts/druid.md)
+    - [DruidOpsRequest](/docs/guides/druid/concepts/druidopsrequest.md)
+
+## How Volume Expansion Process Works
+
+The following diagram shows how KubeDB Ops-manager operator expand the volumes of `Druid` database components. Open the image in a new tab to see the enlarged version.
+
+<figure align="center">
+  <img alt="Volume Expansion process of Druid" src="/docs/guides/druid/volume-expansion/images/druid-volume-expansion.png">
+<figcaption align="center">Fig: Volume Expansion process of Druid</figcaption>
+</figure>
+
+The Volume Expansion process consists of the following steps:
+
+1. At first, a user creates a `Druid` Custom Resource (CR).
+
+2. `KubeDB` Provisioner  operator watches the `Druid` CR.
+
+3. When the operator finds a `Druid` CR, it creates required number of `PetSets` and related necessary stuff like secrets, services, etc.
+
+4. Each PetSet creates a Persistent Volume according to the Volume Claim Template provided in the petset configuration. This Persistent Volume will be expanded by the `KubeDB` Ops-manager operator.
+
+5. Then, in order to expand the volume of the druid data components (ie. Historicals, MiddleManagers) of the `Druid`, the user creates a `DruidOpsRequest` CR with desired information.
+
+6. `KubeDB` Ops-manager operator watches the `DruidOpsRequest` CR.
+
+7. When it finds a `DruidOpsRequest` CR, it halts the `Druid` object which is referred from the `DruidOpsRequest`. So, the `KubeDB` Provisioner  operator doesn't perform any operations on the `Druid` object during the volume expansion process.
+
+8. Then the `KubeDB` Ops-manager operator will expand the persistent volume to reach the expected size defined in the `DruidOpsRequest` CR.
+
+9. After the successful Volume Expansion of the related PetSet Pods, the `KubeDB` Ops-manager operator updates the new volume size in the `Druid` object to reflect the updated state.
+
+10. After the successful Volume Expansion of the `Druid` components, the `KubeDB` Ops-manager operator resumes the `Druid` object so that the `KubeDB` Provisioner  operator resumes its usual operations.
+
+In the next docs, we are going to show a step-by-step guide on Volume Expansion of various Druid database components using `DruidOpsRequest` CRD.

docs/guides/druid/volume-expansion/yamls/deep-storage-config.yaml

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+apiVersion: v1
+kind: Secret
+metadata:
+  name: deep-storage-config
+  namespace: demo
+stringData:
+  druid.storage.type: "s3"
+  druid.storage.bucket: "druid"
+  druid.storage.baseKey: "druid/segments"
+  druid.s3.accessKey: "minio"
+  druid.s3.secretKey: "minio123"
+  druid.s3.protocol: "http"
+  druid.s3.enablePathStyleAccess: "true"
+  druid.s3.endpoint.signingRegion: "us-east-1"
+  druid.s3.endpoint.url: "http://myminio-hl.demo.svc.cluster.local:9000/"
+

docs/guides/druid/volume-expansion/yamls/druid-cluster.yaml

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+apiVersion: kubedb.com/v1alpha2
+kind: Druid
+metadata:
+  name: druid-cluster
+  namespace: demo
+spec:
+  version: 28.0.1
+  deepStorage:
+    type: s3
+    configSecret:
+      name: deep-storage-config
+  topology:
+    historicals:
+      replicas: 1
+      storage:
+        accessModes:
+          - ReadWriteOnce
+        resources:
+          requests:
+            storage: 1Gi
+        storageClassName: standard
+    middleManagers:
+      replicas: 1
+      storage:
+        accessModes:
+          - ReadWriteOnce
+        resources:
+          requests:
+            storage: 1Gi
+        storageClassName: standard
+    routers:
+      replicas: 1
+  deletionPolicy: Delete
+

docs/guides/druid/volume-expansion/yamls/volume-expansion-ops.yaml

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+apiVersion: ops.kubedb.com/v1alpha1
+kind: DruidOpsRequest
+metadata:
+    name: dr-volume-exp
+    namespace: demo
+spec:
+    type: VolumeExpansion
+    databaseRef:
+        name: druid-cluster
+    volumeExpansion:
+        historicals: 3Gi
+        middleManagers: 2Gi
+        mode: Online