demo applications add info how to disable them

[astroidex]

Describe your feature request

• Add info that cache has to be deleted when changed published: false and that the application is still visible for the administrator user
• add info that the demo applications can also be deleted
• see de/en https://doc.mapbender.org/en/customization/yaml.html?highlight=published%20false#yaml-application-files
• see email conversation mailing list