Update pages about the various communication of the project /advertise matrix

We need to do an update on the various pages about where we communicate in the project and advertise to new matrix room
(using eclipse server directly: https://chat.eclipse.org/#/room/#gemoc-studio:matrix.eclipse.org or via existing account in the matrix federation `#gemoc-studio:matrix.eclipse.org` https://matrix.to/#/#gemoc-studio:matrix.eclipse.org)

- [ ] Forum (https://www.eclipse.org/forums/index.php/f/369/)
- [x] GEMOC initiative mailing list (gemoc-initiative@inria.fr and all@gemoc.org)
- [x] Eclipse project description https://projects.eclipse.org/projects/modeling.gemoc/contact
- [ ] GEMOC Studio web page (https://gemoc.org/contact.html)