Massimiliano started an experimental (so some of its features may change) wiki at https://texmacs.github.io/notes/. People can contribute through pull requests at https://github.com/texmacs/notes.
If the experiment is successful in a few weeks it could be linked to from the main TeXmacs webpage and so become more widely known.
I hope that can be, together with the mailing list and the forum, a place where users can contribute to the development of the software.
The discourse forum is notifying me as I write of two posts relevant to his topic, so I link to them here