readme, about: Update information
I updated some information in the README and About window. For the README, I linked to Matrix, as IRC was retired.
As for the About window, I added a support URL and pointed to Discourse for those who may want some technical support. I also added a dedicated designers section to give more accurate information.
Just a proposal: we should remove the current date in the copyright section (–2022), as copyright implies "and later". That way, we won't have to update it every year.