Move embedded screenshot from to-be-retired wiki.gnome.org to GitLab repository
Per https://discourse.gnome.org/t/wiki-gnome-org-retirement-feedback-wanted/17314/28
Image should probably be moved in GitLab code repository and called via https://gitlab.gnome.org/GNOME/$reponame/-/raw/$branchname/path/to/image/$imagename.png
./geary/README.md:![Geary icon](https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-3-32-256-logo.png)
./geary/README.md:![Geary displaying a conversation](https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-3-32-main-window.png)
./geary/desktop/org.gnome.Geary.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-40-main-window.png</image>
./geary/desktop/org.gnome.Geary.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-40-conversation.png</image>
./geary/desktop/org.gnome.Geary.appdata.xml.in.in: <image>https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-3-36-composer.png</image>