docs: Fix the Git links for the example apps

The links to the repository's web UI still refer to the old
git.gnome.org cgit UI, and to the master branch; we should be using
GitLab and the gtk-3-22 branch instead.
1 job for gtk-3-22 in 4 minutes and 58 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Build
passed #22631
build

00:04:58