Add druid volume expansion docs

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

docs/guides/druid/autoscaler/_index.md

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+---
+title: Autoscaling
+menu:
+  docs_{{ .version }}:
+    identifier: guides-druid-autoscaler
+    name: Autoscaling
+    parent: guides-druid
+    weight: 46
+menu_name: docs_{{ .version }}
+---

docs/guides/druid/autoscaler/compute/_index.md

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+---
+title: Compute Autoscaling
+menu:
+  docs_{{ .version }}:
+    identifier: guides-druid-autoscaler-compute
+    name: Compute Autoscaling
+    parent: guides-druid-autoscaler
+    weight: 46
+menu_name: docs_{{ .version }}
+---

docs/guides/druid/autoscaler/compute/overview.md

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+---
+title: Druid Compute Autoscaling Overview
+menu:
+  docs_{{ .version }}:
+    identifier: guides-druid-autoscaler-compute-overview
+    name: Overview
+    parent: guides-druid-autoscaler-compute
+    weight: 10
+menu_name: docs_{{ .version }}
+section_menu_id: guides
+---
+
+> New to KubeDB? Please start [here](/docs/README.md).
+
+# Druid Compute Resource Autoscaling
+
+This guide will give an overview on how KubeDB Autoscaler operator autoscales the database compute resources i.e. cpu and memory using `kafkaautoscaler` crd.
+
+## Before You Begin
+
+- You should be familiar with the following `KubeDB` concepts:
+  - [Druid](/docs/guides/kafka/concepts/kafka.md)
+  - [DruidAutoscaler](/docs/guides/kafka/concepts/kafkaautoscaler.md)
+  - [DruidOpsRequest](/docs/guides/kafka/concepts/kafkaopsrequest.md)
+
+## How Compute Autoscaling Works
+
+The following diagram shows how KubeDB Autoscaler operator autoscales the resources of `Druid` database components. Open the image in a new tab to see the enlarged version.
+
+<figure align="center">
+  <img alt="Compute Auto Scaling process of Druid" src="/docs/images/day-2-operation/kafka/kf-compute-autoscaling.svg">
+<figcaption align="center">Fig: Compute Auto Scaling process of Druid</figcaption>
+</figure>
+
+The Auto Scaling process consists of the following steps:
+
+1. At first, a user creates a `Druid` Custom Resource Object (CRO).
+
+2. `KubeDB` Provisioner operator watches the `Druid` CRO.
+
+3. When the operator finds a `Druid` CRO, it creates required number of `PetSets` and related necessary stuff like secrets, services, etc.
+
+4. Then, in order to set up autoscaling of the various components (ie. Coordinators, Overlords, Historicals, MiddleManagers, Brokers, Routers) of the `Druid` cluster the user creates a `DruidAutoscaler` CRO with desired configuration.
+
+5. `KubeDB` Autoscaler operator watches the `DruidAutoscaler` CRO.
+
+6. `KubeDB` Autoscaler operator generates recommendation using the modified version of kubernetes [official recommender](https://github.com/kubernetes/autoscaler/tree/master/vertical-pod-autoscaler/pkg/recommender) for different components of the database, as specified in the `DruidAutoscaler` CRO.
+
+7. If the generated recommendation doesn't match the current resources of the database, then `KubeDB` Autoscaler operator creates a `DruidOpsRequest` CRO to scale the database to match the recommendation generated.
+
+8. `KubeDB` Ops-manager operator watches the `DruidOpsRequest` CRO.
+
+9. Then the `KubeDB` Ops-manager operator will scale the database component vertically as specified on the `DruidOpsRequest` CRO.
+
+In the next docs, we are going to show a step by step guide on Autoscaling of various Druid database components using `DruidAutoscaler` CRD.