postgres: add migration guide for self-managed sources via remote replica

docs/guides/postgres/remote-replica/migration-yamls/pg-mig-standalone.yaml

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+apiVersion: kubedb.com/v1
+kind: Postgres
+metadata:
+  name: pg-mig
+  namespace: demo
+spec:
+  clientAuthMode: md5
+  standbyMode: Hot
+  replicas: 1
+  storage:
+    accessModes: [ReadWriteOnce]
+    resources:
+      requests:
+        storage: 10Gi          # must hold the full source database plus WAL
+  storageType: Durable
+  deletionPolicy: Halt
+  version: "17.4"              # must match the source's MAJOR version

docs/guides/postgres/remote-replica/migration-yamls/pg-mig.yaml

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+apiVersion: kubedb.com/v1
+kind: Postgres
+metadata:
+  name: pg-mig
+  namespace: demo
+spec:
+  remoteReplica:
+    sourceRef:
+      name: source-pg          # the AppBinding above, in THIS cluster
+      namespace: demo
+  authSecret:
+    name: source-pg-auth       # during the warm phase: the migration user's credentials
+  clientAuthMode: md5
+  standbyMode: Hot
+  replicas: 1
+  storage:
+    accessModes: [ReadWriteOnce]
+    resources:
+      requests:
+        storage: 10Gi          # must hold the full source database plus WAL
+  storageType: Durable
+  deletionPolicy: Halt
+  version: "17.4"              # must match the source's MAJOR version

docs/guides/postgres/remote-replica/migration-yamls/source-appbinding.yaml

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+# Hand-crafted AppBinding for a source PostgreSQL that is NOT managed by KubeDB.
+# Edit: service.name (source address), secret credentials, and sslmode.
+apiVersion: v1
+kind: Secret
+metadata:
+  name: source-pg-auth
+  namespace: demo
+stringData:
+  username: migrator          # the replication user the source DBA created for you
+  password: "<migrator-password>"
+type: kubernetes.io/basic-auth
+---
+apiVersion: appcatalog.appscode.com/v1alpha1
+kind: AppBinding
+metadata:
+  name: source-pg
+  namespace: demo
+  labels:
+    app.kubernetes.io/name: postgreses.kubedb.com
+spec:
+  clientConfig:
+    service:
+      name: 10.2.0.30         # source address reachable from THIS cluster
+      path: /
+      port: 5432               # honored: set to the source's actual port
+      query: sslmode=disable  # or verify-ca with a tlsSecret carrying the SOURCE's CA
+      scheme: postgresql
+  secret:
+    apiGroup: ""
+    kind: Secret
+    name: source-pg-auth
+  type: kubedb.com/postgres
+  version: "17.4"