Exclude autoscaling docs from release docs (#51190)

Since autoscaling is currently only under development, this commit
causes the autoscaling docs to be excluded any time that release docs
are being built.

docs/reference/index.asciidoc

@@ -40,8 +40,12 @@ include::ingest.asciidoc[]
 
 include::ilm/index.asciidoc[]
 
+ifeval::["{release-state}"=="unreleased"]
+
 include::autoscaling/index.asciidoc[]
 
+endif::[]
+
 include::sql/index.asciidoc[]
 
 include::monitoring/index.asciidoc[]