Suggest that folks enable DMARC reporting

SNM supports DMARC reporting, but it's disabled by default. For email
greybeards, that's fine, but I think it would be useful to teach email newbies (as I was a few
months ago) that this is something you should seriously consider
enabling.

I opted to put this in a new "Advanced Configurations" section that
points experienced mailserver admins to our howto guides, and newbies to
a couple of important things.

refs: https://github.com/NixOS/infra/pull/635

docs/advanced-configurations.rst

@@ -0,0 +1,14 @@
+Advanced Configurations
+=======================
+
+Congratulations on completing the `Setup Guide <setup-guide.html>`_!
+
+If you're an experienced mailserver admin, then you probably know what you want
+to do next. Our How-to guides (accessible in the navigation sidebar)
+might help you accomplish your goals. If not, consider contributing a guide!
+
+If this is your first mailserver, consider the following:
+
+- Set up `backups <backup-guide.html>`_.
+- Enable `DMARC reporting <options.html#mailserver-dmarcreporting>`_ to be a
+  good citizen in the mail ecosystem.