(DOCSP-15593) Add Lookup Fields to backing agg pipeline doc (#471)

Author: zach-carr

source/aggregation-pipeline-generation.txt

@@ -22,6 +22,7 @@ The pipeline constructed by |charts| consists of the following segments in the f
 #. `Dashboard Filters <https://docs.mongodb.com/charts/saas/dashboard-filtering/#filter-dashboards-by-field-values>`_
 #. `Query Bar <https://docs.mongodb.com/charts/saas/filter-chart-results/#filter-your-data-using-the-query-bar>`_
 #. `Embedding Filters <https://docs.mongodb.com/charts/saas/embedded-chart-options/#embedded-chart-options>`_
+#. `Lookup Fields <https://docs.mongodb.com/charts/saas/add-lookup-field/>`_
 #. `Calculated Fields <https://docs.mongodb.com/charts/saas/calculated-fields/>`_
 #. `Chart Filter Pane <https://docs.mongodb.com/charts/saas/filter-chart-results/#create-filters-for-your-data>`_
 #. `Encoding <https://docs.mongodb.com/charts/saas/encoding-channels/>`_