DOCSP-18353 Add documentation for merge stage (#195)

* DOCSP-18353 Add documentation for merge stage

* DOCSP-18353 updates for review feedback

* DOCSP-18353 updates for review feedback

* DOCSP-18353 updates for review feedback

* DOCSP-18353 added note

* DOCSP-18353 address minor nits:

* DOCSP-18353 clarifying sort stage

Author: kanchana-mongodb

source/reference/pipeline/aggr-pipeline.txt

@@ -15,6 +15,7 @@ introduces for the following :manual:`pipeline
 
 - :ref:`adl-collstats-stage`
 - :ref:`adl-lookup-stage`
+- :ref:`adl-merge-stage`
 - :ref:`adl-out-stage`
 
 .. toctree::
@@ -23,6 +24,7 @@ introduces for the following :manual:`pipeline
 
    /reference/pipeline/collstats
    /reference/pipeline/lookup-stage
+   /reference/pipeline/merge
    /reference/pipeline/out
    $queryHistory </query/view-query-history/>
    /reference/pipeline/sql