tps-frontend: Fix opening documentation causing window to freeze

Since af7421d9 we capture the stderr of the tails-documentation command (to show it in the error message if the command fails). That causes the subprocess.run() call to block until the stderr file descriptor was closed. Because tails-documentation calls gtk-launch which in turn passes the stderr file descriptor on to the launched application, the file descriptor is not closed until the launched application is closed.

This commit uses systemd-cat to connect the stderr (and stdout) of gtk-launch to the journal instead, which has the effect that the stderr file descriptor of tails-documentation is not passed on to gtk-launch and is therefore closed immediately when tails-documentation exits.

Closes #20438 (closed)