(DOCSP-9903): Mention Online Archive on ttl indexes page

Author: jeff-allen-mongo

source/core/index-ttl.txt

@@ -14,6 +14,15 @@ TTL Indexes
    :depth: 1
    :class: singlecol
 
+.. note::
+
+   If you are removing documents to save on storage costs, consider
+   :atlas:`Online Archive </online-archive/manage-online-archive>` in
+   `MongoDB Atlas <https://www.mongodb.com/cloud?jmp=docs>`__. Online
+   Archive automatically archives infrequently accessed data to
+   fully-managed S3 buckets for cost-effective data
+   tiering.
+
 TTL indexes are special single-field indexes that MongoDB can use to
 automatically remove documents from a collection after a certain amount
 of time or at a specific clock time. Data expiration is useful for certain types of information