Add documentation for cdi populators

doc/cdi-populators.md

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+# CDI Volume Populators
+
+## Recommended knowledge
+
+What are [populators](https://kubernetes.io/blog/2022/05/16/volume-populators-beta/)
+
+## Introduction
+CDI volume populators are CDI's implementation of importing/uploading/cloning data to PVCs using the new `dataSourceRef` field. New controllers and custom resources for each population method were introduced.
+
+**Benefits of the new API**
+* Native synchronization with Kubernetes - this is Kubernetes way of populating PVCs. Once PVC is bound we know it is populated (Before introducing populators, the PVC was bound the moment the datavolume created it, and the population progress was monitored via the datavolume)
+* Use PVCs directly and have them populated without the need for datavolumes.
+* Can use one population definition for multiple PVCs - create 1 CR defining population source and use it for any PVC.
+* Better compatibility with existing backup and restore solutions:
+    * Standalone PVC should be compatible with all solutions.
+    * Datavolumes using the populators should be compatible with most solutions, for example Metro DR and [Gitops](https://www.redhat.com/en/topics/devops/what-is-gitops#:~:text=GitOps%20uses%20Git%20repositories%20as,set%20for%20the%20application%20framework.) - when restoring datavolume manifest will be applied, the datavolume will create the PVC that will bind immediately to the PV waiting for it.
+    * [Kubevirt](https://github.com/kubevirt/kubevirt) VMs with PVCs/datavolumetemplate should also be compatible with most solutions.
+* Integration with [Kubevirt](https://github.com/kubevirt/kubevirt) with WaitForFirstConsumer(WFFC) storage class is simpler and does not require a [doppelganger pod](https://github.com/kubevirt/kubevirt/blob/main/docs/localstorage-disks.md#the-problem) for the start of the VM.
+
+
+## Using the populators
+
+We introduced new controllers for each population method. Each controller supports a matching CRD which provides the needed information for the population.
+Example for an instance of VolumeImportSource CRD:
+
+```yaml
+apiVersion: cdi.kubevirt.io/v1beta1
+kind: VolumeImportSource
+metadata:
+  name: my-import-source
+spec:
+  source:
+      http:
+         url: "https://download.cirros-cloud.net/0.4.0/cirros-0.4.0-x86_64-disk.img"
+```
+
+### Using populators with PVCs
+User can create a CR and PVCs specifying the CR in the `DataSourceRef` field and those will be handled by the matching populator controller.
+
+#### Import
+Example for PVC which use the VolumeImportSource above that will be handled by the import populator:
+```yaml
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name:  my-pvc
+spec:
+  dataSourceRef:
+    apiGroup: cdi.kubevirt.io
+    kind: VolumeImportSource
+    name: my-import-source
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: 10Gi
+```
+
+Above PVC will trigger the reconcile of the import populator controller.
+The controller will create a matching temporary PVC with the appropriate annotations, which will get bound and populated.
+Once the temporary PVC population is done, the PV will be rebound to the original PVC completing the population process.
+
+#### Upload
+Example of VolumeUploadSource and a PVC that will be handled by the upload populator:
+```yaml
+apiVersion: cdi.kubevirt.io/v1beta1
+kind: VolumeUploadSource
+metadata:
+  name: my-upload-source
+spec:
+   contentType: kubevirt
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name:  my-pvc
+spec:
+  dataSourceRef:
+    apiGroup: cdi.kubevirt.io
+    kind: VolumeUploadSource
+    name: my-upload-source
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: 10Gi
+```
+After creating the VolumeUploadSource and PVC, you can start the upload to the pvc as describe in the [upload doc](upload.md).