📦 NEW: Add chat (Matrix/IRC) section to contact page

I've added a section about how to reach us via chat (Matrix/IRC) to the contact page.

Please review and tell me what you think. (Especially the language part as I'm neither French nor English native speaker. 🙈 )
cc @debeule @afontain @floure

resolves #19 (closed) #12 (closed)