wip: Add examples for mail on unit failure

Is this a suitable place to put them?

Closes https://gitlab.torproject.org/tpo/network-health/sbws/-/issues/40178

debian/examples/sbws_generate.d/mail_on_failure.conf

+OnFailure=status-email-user@%n.service
\ No newline at end of file

debian/examples/status_mail.sh

+#!/bin/bash
+
+/usr/sbin/sendmail -t <<ERRMAIL
+To: $1
+From: systemd <root@$HOSTNAME>
+Subject: $2
+Content-Transfer-Encoding: 8bit
+Content-Type: text/plain; charset=UTF-8
+
+$(systemctl status --full "$2")
+ERRMAIL
\ No newline at end of file

debian/examples/status_mail_user@.service

+[Unit]
+Description=status email for %i to user
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/status_mail.sh sbws@localhost %i
+User=nobody
+Group=systemd-journal

debian/sbws.examples

+debian/examples/*