homelab/kubernetes: cloudnative-pg how to copy existing db

- add documentation regarding how to copy existing DB using
  cloudnative-pg

kubernetes/cloud-native-pg/pg-main-backup.yaml

@@ -0,0 +1,10 @@
+apiVersion: postgresql.cnpg.io/v1
+kind: ScheduledBackup
+metadata:
+  name: pg-main-backup 
+spec:
+  immediate: true
+  schedule: "0 0 0 * * *"  # At midnight every day
+  backupOwnerReference: self
+  cluster:
+    name: master-postgres