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.

docs/index.rst

@@ -14,6 +14,7 @@ Welcome to NixOS Mailserver's documentation!
    :maxdepth: 2
 
    setup-guide
+   advanced-configurations
    howto-develop
    faq
    release-notes

docs/setup-guide.rst

@@ -236,3 +236,8 @@ Besides that, you can send an email to
 score, and let `mxtoolbox.com <http://mxtoolbox.com/>`__ take a look at
 your setup, but if you followed the steps closely then everything should
 be awesome!
+
+Next steps (optional)
+~~~~~~~~~~~~~~~~~~~~~
+
+Take a look through our `Advanced Configurations <advanced-configurations.html>`_.