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

[dvojtise]

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/)
• GEMOC initiative mailing list (gemoc-initiative@inria.fr and all@gemoc.org)
• Eclipse project description https://projects.eclipse.org/projects/modeling.gemoc/contact
• GEMOC Studio web page (https://gemoc.org/contact.html)