[cherry-pick] Add auto-backup doc + Restructure docs (#618)

docs/auto-backup/examples/backupblueprint.yaml

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+apiVersion: stash.appscode.com/v1beta1
+kind: BackupBlueprint
+metadata:
+  name: postgres-backup-template
+spec:
+  # ============== Blueprint for Repository ==========================
+  backend:
+    gcs:
+      bucket: stash-testing
+      prefix: stash-backup/${TARGET_NAMESPACE}/${TARGET_APP_RESOURCE}/${TARGET_NAME}
+    storageSecretName: gcs-secret
+  # ============== Blueprint for BackupConfiguration =================
+  task:
+    name: postgres-backup-11.9.0-v4
+  schedule: "*/5 * * * *"
+  retentionPolicy:
+    name: 'keep-last-5'
+    keepLast: 5
+    prune: true

docs/auto-backup/examples/sample-pg-1.yaml

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+apiVersion: kubedb.com/v1alpha2
+kind: Postgres
+metadata:
+  name: sample-postgres-1
+  namespace: demo
+  annotations:
+    stash.appscode.com/backup-blueprint: postgres-backup-template
+spec:
+  version: "11.2-v1"
+  storageType: Durable
+  storage:
+    storageClassName: "standard"
+    accessModes:
+    - ReadWriteOnce
+    resources:
+      requests:
+        storage: 1Gi
+  terminationPolicy: Delete

docs/auto-backup/examples/sample-pg-2.yaml

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+apiVersion: kubedb.com/v1alpha2
+kind: Postgres
+metadata:
+  name: sample-postgres-2
+  namespace: demo-2
+  annotations:
+    stash.appscode.com/backup-blueprint: postgres-backup-template
+    stash.appscode.com/schedule: "*/3 * * * *"
+spec:
+  version: "11.2-v1"
+  storageType: Durable
+  storage:
+    storageClassName: "standard"
+    accessModes:
+    - ReadWriteOnce
+    resources:
+      requests:
+        storage: 1Gi
+  terminationPolicy: Delete

docs/auto-backup/examples/sample-pg-3.yaml

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+apiVersion: kubedb.com/v1alpha2
+kind: Postgres
+metadata:
+  name: sample-postgres-3
+  namespace: demo-3
+  annotations:
+    stash.appscode.com/backup-blueprint: postgres-backup-template
+    params.stash.appscode.com/args: --no-owner --clean
+spec:
+  version: "11.2-v1"
+  storageType: Durable
+  storage:
+    storageClassName: "standard"
+    accessModes:
+    - ReadWriteOnce
+    resources:
+      requests:
+        storage: 1Gi
+  terminationPolicy: Delete