DOCSP-34286 -- add page to json.txt toc

Author: jvincent-mongodb

source/reference/json.txt

@@ -142,4 +142,5 @@ see :ref:`example-alert-config-file`.
    Cloud Backup Schedule </reference/json/cloud-backup-schedule-config-file>
    Atlas Data Federation </reference/json/data-federation-config-file>
    Atlas Search Index </reference/json/search-index-config-file>
+   Online Archive </reference/json/file-options-online-archive>
    Alert </reference/json/alert-config-file>