Included pages is a plugin for dotclear. It aims to provide a simple way to serve HTML templates and PHP scripts.
The main developement has been made by Pep - https://nrkn.fr/ and maintained over the years by nikrou - https://www.nikrou.net/
I will now try to maintain that plugin so that it can be used in recent dotclear releases.
You can install the plugin from your dotclear administration area.
If you have ideas to improve that plugin, feel free to add an issue or even better send me a pull request. If you find a bug, fill an issue.