Help user to setup .asciidoctorconfig for attributes

[ahus1]

A user might now know about the .asciidoctorconfig file that allow her/him to configuration attributes with values to be replaced in the preview.

A notification should provide a pop-up with additional information and/or an automatic setup of the file when it detect attributes for example in includes and images, for example specific to spring-boot-rest-docs as experienced by one of the plugin's users in #310