This is where Concept Three is explained.
This is the first topic from the Root page of this Welcome:
(This page demonstrates how to contribute to an existing Welcome.
It was contributed using the org.eclipse.ui.intro.configExtension
extension point. It also
demonstrates how to use the include and the dynamicContent intro elements to
include external XHTML content and to create dynamic XHTML content,
respectively. Check the XHTML source for this page for usage of intro elements.)