docs: add collections API endpoint files (Phase 2 completion)

docs/api/collections/delete_shelf_mapping.md

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+# Delete Shelf Mapping
+
+Remove a collection-to-shelf mapping for a device.
+
+**Endpoint**: `DELETE /api/devices/{deviceId}/collections/{collectionId}`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| deviceId | string (UUID) | Yes | Device UUID |
+| collectionId | string (UUID) | Yes | Collection UUID |
+
+## Response (204 No Content)
+
+Shelf mapping deleted successfully. No response body.
+
+## Notes
+
+- Deleting a mapping does NOT delete the collection
+- Deleting a mapping does NOT delete books from the device
+- It only removes the association between the collection and device shelf
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Authentication required |
+| 404 | Device or collection not found |
+| 500 | Internal server error |
+
+## Try It Out