As can be seen at https://maven.apache.org/guides/mini/guide-site.html we can put all the content we create under
scr/site/apt/
Take a look at the repository to get a feel for what needs to be there. A quick guide for APT can be found at https://maven.apache.org/doxia/references/apt-format.html. Notice that I never used APT before this little project and in fact it is not that hard once you just dive into it.