add split index reference in indices.asciidoc

Relates to #26931

docs/reference/indices.asciidoc

@@ -71,6 +71,8 @@ include::indices/open-close.asciidoc[]
 
 include::indices/shrink-index.asciidoc[]
 
+include::indices/split-index.asciidoc[]
+
 include::indices/rollover-index.asciidoc[]
 
 include::indices/put-mapping.asciidoc[]