documentation: change order of pipelines topics (#4427)

Author: stacicho

doc/workflows/index.rst

@@ -7,7 +7,7 @@ The SageMaker Python SDK supports managed training and inference for a variety o
 .. toctree::
     :maxdepth: 2
 
+    pipelines/index
     airflow/index
     step_functions/index
-    pipelines/index
     lineage/index