Add initial docs for mailmon

much of which is just stubs for now...

docs/base/mailmon.rst

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+
+Mail Monitoring
+===============
+
+Rattail has a basic "Mail Monitoring" (mailmon) service which can
+watch an IMAP folder, and perform actions on any messages which
+appear.  
+
+Note that one of those actions, presumably the last, should be to
+*move* the message into a different IMAP folder, or perhaps delete it
+outright.  This is to avoid re-processing the same message over and
+over.  The IMAP folder being watched is essentially treated as a
+"queue" and everything in it will be processed.
+
+Example config for this looks something like (in ``mailmon.conf``):
+
+.. code-block:: ini
+
+   [rattail.mailmon]
+   monitor = myfolder
+
+   myfolder.imap.server = mail.example.com
+   myfolder.imap.username = myuser
+   myfolder.imap.password = mypass
+   myfolder.imap.folder = INBOX.Incoming-Messages
+   myfolder.actions = download, move
+   myfolder.action.download.func = rattail.mailmon.actions:download_message
+   myfolder.action.download.args = /srv/envs/poser/app/work/mailmon/myfolder
+   myfolder.action.download.kwarg.locking = true
+   myfolder.action.move.func = rattail.mailmon.actions:move_message
+   myfolder.action.move.args = INBOX.Processed-Messages
+
+In the above example we are watching the "Incoming-Messages" folder,
+which is a subfolder of the INBOX itself.  Any messages which appear
+there will be downloaded to local file system, and then moved to the
+"Processed-Messages" IMAP folder on the server.
+
+Running the command is usually done via supervisor (so as to make it
+behave as a proper service), but the command line itself looks like:
+
+.. code-block:: sh
+
+   cd /srv/envs/poser
+   sudo -u rattail bin/rattail -c app/mailmon.conf mailmon start