Add documentation for failure stores.

manage-data/toc.yml

@@ -20,6 +20,7 @@ toc:
               - file: data-store/data-streams/run-downsampling-using-data-stream-lifecycle.md
               - file: data-store/data-streams/reindex-tsds.md
           - file: data-store/data-streams/logs-data-stream.md
+          - file: data-store/data-streams/failure-store.md
       - file: data-store/mapping.md
         children:
           - file: data-store/mapping/dynamic-mapping.md