Add collection migration example and guide

examples/go-library/prepare/collections/review/collection.yaml

@@ -0,0 +1,17 @@
+backends:
+  offline:
+    endpoint: https://review.example.invalid/v1
+profiles:
+  audit:
+    backend: review/offline
+    model: review-model
+prompts:
+  check:
+    default_profile: review/audit
+    description: Inspect a review item without contacting a model provider.
+    messages:
+      - role: user
+        content: Review the supplied synthetic example.
+    output:
+      format: text
+      validation_mode: none