Add index term for backup manifest in documentation.

Author: Fujii Masao
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/951743d0-fd7e-8e2b-d489-1368a58b7304@oss.nttdata.com

doc/src/sgml/backup-manifest.sgml

@@ -3,6 +3,10 @@
 <chapter id="backup-manifest-format">
  <title>Backup Manifest Format</title>
 
+  <indexterm>
+   <primary>Backup Manifest</primary>
+  </indexterm>
+
   <para>
    The backup manifest generated by <xref linkend="app-pgbasebackup" /> is
    primarily intended to permit the backup to be verified using