Document how to troubleshoot in (debian) production

docs/source/install-on-debian.rst

@@ -166,6 +166,9 @@ Set up the configuration file
    it.
 #. In the config-file, set ``devmode`` in the ``[howitz]`` section to
    ``false``, no quotes.
+#. Because of ``devmode`` being ``false`` you also need to set ``listen`` to
+   ``"127.0.0.1"`` and ``port`` to ``5000`` in the same section as ``devmode``,
+   so that gunicorn can find Howitz.
 #. Eventually you will probably wish to lower the log-level. In the
    config-file, set ``level`` in the ``[logging.root]`` section to ``"INFO"``,
    note the quotes.

docs/source/troubleshooting-on-debian.rst

@@ -0,0 +1,24 @@
+=========================
+Troubleshooting on Debian
+=========================
+
+Visit the URL of the site. If there is a "50x"-error, and the logs are not
+exported to an external log tool you need to log in to the console of the site.
+
+Check systemctl status
+======================
+
+Run ``sudo systemctl status howitz-gunicorn.service``. If it says "disabled" or
+there is red text, Howitz didn't start properly or has crashed. Go to
+`Check journalctl`_
+
+Check journalctl
+================
+
+Run `journalctl --since="-1h" -u howitz-gunicorn.service`. This will show the
+gunicorn log for the last hour.
+
+If there is an exception mentioning "pydantic", you might have forgotten to
+configure everything. Remember that if ``devmode`` is ``false``, then
+``listen`` must be set to an IP address and ``port`` must be set to a port
+number.