[doc] Add informations on the generated secret plugin-pgbackrest-xxxxx

[pchovelon]

The Deployment has a template part that olds configuration and especially a Secret named
plugin-pgbackrest-xxxxx. It holds the sidecar image (BASE64) to use.

  template:
    metadata:
      labels:
        app: pgbackrest-controller
    spec:
[...]
        env:
        - name: SIDECAR_IMAGE
          valueFrom:
            secretKeyRef:
              key: SIDECAR_IMAGE
              name: plugin-pgbackrest-66hkbh8k26

Nothing in the documentation talk about that. We should add explanations on what is its purpose.