Add mapped_pages section to failure-store docs (elastic#3437)

couple of missing mapped pages (this was an 8.19/9.1 feature)

Author: shainaraskas

manage-data/data-store/data-streams/failure-store-recipes.md

@@ -1,4 +1,6 @@
 ---
+mapped_pages:
+  - https://www.elastic.co/guide/en/elasticsearch/reference/current/failure-store-recipes.html
 applies_to:
   stack: ga 9.1
   serverless: ga

manage-data/data-store/data-streams/failure-store.md

@@ -1,4 +1,6 @@
 ---
+mapped_pages:
+  - https://www.elastic.co/guide/en/elasticsearch/reference/current/failure-store.html
 applies_to:
   stack: ga 9.1
   serverless: ga