In the end we must work together to create a new skin (https://maven.apache.org/skins/index.html has current examples). Now a single (sub)project has 1 page layout, often a menu (top or left) and the rest is used for content. I can imagine that general documentation / landing pages get their own template. That's all possible, but must be developed by us.
Be aware that any content somehow fit. The template will have some placeholders with e.g menuitems and the actual content.
Skins are very easy to apply to maven generated websites, so a good skin could conquer the world :)
https://maven.apache.org/plugins/maven-site-plugin/examples/creating-content.html (and other examples)