[DOC] Add beta banner to main page of Transforms (#53399)

Author: lucabelluccini

docs/reference/transform/index.asciidoc

@@ -2,6 +2,8 @@
 [[transforms]]
 == Transforming data
 
+beta[]
+
 // tag::transform-intro[]
 {transforms-cap} enable you to convert existing {es} indices into summarized
 indices, which provide opportunities for new insights and analytics.
@@ -25,4 +27,4 @@ include::api-quickref.asciidoc[]
 include::ecommerce-tutorial.asciidoc[]
 include::examples.asciidoc[]
 include::troubleshooting.asciidoc[]
-include::limitations.asciidoc[]
+include::limitations.asciidoc[]