Struts2 plugins (jQuery plugin, Bootstrap plugin, etc...) have the purpose of helping you integrating different technologies in a transparent way, without forcing you to immediately learn those technologies.
You write a simple Struts tag and, thanks to a special theme (the bootstrap
theme, in this case),
the HTML generated will be Bootstrap-compliant.
In order to do this, the plugin has already in it the latest version of Bootstrap (the 3.3.7, as you can see on GitHub), so, to answer your question
Since in the Struts2 Bootstrap Plugin Showcase the list-group does not
exist, should I need to include bootstrap-3.3.7 in my project ?
If the tag you want has not been implemented, you can still manually write its raw (bootstrap-compliant) HTML, exploiting the Bootstrap library bundled with the plugin.
You have no need to include that library again, it is already in your project.
Note: the latest version of the plugin is 2.5.1.