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

docs/api/collections/remove_auto_assign_rule.md

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+# Remove Auto-Assign Rule
+
+Remove an automatic book assignment rule from a collection.
+
+**Endpoint**: `DELETE /api/collections/{collectionId}/rules/{ruleId}`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| collectionId | string (UUID) | Yes | Collection UUID |
+| ruleId | string (UUID) | Yes | Rule UUID |
+
+## Response (204 No Content)
+
+Rule deleted successfully. No response body.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Authentication required |
+| 404 | Collection or rule not found |
+| 500 | Internal server error |
+
+## Try It Out