re-adds reference/mongo.txt (#1907)

Author: jmd-mongo

source/reference.txt

@@ -22,6 +22,10 @@ Reference
    Documentation of JavaScript methods and helpers in
    :binary:`~bin.mongosh`.
 
+:doc:`/reference/mongo`
+   Documentation of JavaScript methods and helpers in
+   the legacy :binary:`~bin.mongo` shell.
+
 :doc:`/reference/program`
    Documentation of :binary:`~bin.mongod` and :binary:`~bin.mongos`
    and all other tools distributed with MongoDB.
@@ -97,6 +101,7 @@ Reference
    /reference/operator
    /reference/command
    /reference/method
+   /reference/mongo
    /reference/program
    /reference/configuration-options
    /reference/parameters