add link to demos/stacks customization section

Author: xeniape

docs/modules/demos/pages/index.adoc

@@ -44,3 +44,5 @@ Each demo is constructed in the following way:
   ** `stackableOperators` are all the operators that are installed as part of the demo
   ** `manifests` contains all the manifests needed to setup the products and third-party tools
   ** `parameters` is a list of values used in the manifest files, for example as credentials or other settings
+
+For more information on how to add custom demos/stacks, refer to the xref:management:stackablectl:customization/index.adoc[Customization section of SDP Management,window=_blank].