docs: Add page for the Debian package

(cherry picked from commit 9f6610a144)

docs/debian.rst

@@ -0,0 +1,92 @@
+.. _debian:
+
+**************
+Debian package
+**************
+
+The Mopidy Debian package is available from `apt.mopidy.com
+<http://apt.mopidy.com/>`__ as well as from Debian, Ubuntu and other
+Debian-based Linux distributions.
+
+
+Installation
+============
+
+See the Debian/Ubuntu section in the :ref:`installation` section.
+
+
+Running as a system service by default
+======================================
+
+The Debian package comes with an init script and, by default, it starts Mopidy
+as a system service running as the ``mopidy`` user which is created by the
+package.
+
+If you've installed Mopidy from the Debian package and don't want to use the
+init script to run Mopidy as a system service, but instead just run Mopidy
+manually using your own user, you need to disable the init script and stop
+Mopidy by running::
+
+    sudo update-rc.d -f mopidy remove
+    sudo service mopidy stop
+
+
+Differences between when running as a system service
+====================================================
+
+If you want to run Mopidy using the init script, there's a few differences
+from a regular Mopidy setup you'll want to know about.
+
+- All configuration is in :file:`/etc/mopidy`, not in your user's home
+  directory. The main configuration file is :file:`/etc/mopidy/mopidy.conf`.
+  You can do all your changes in this file.
+  
+- Mopidy extensions installed from Debian packages will sometimes install
+  additional configuration files in :file:`/etc/mopidy/extensions.d/`. These
+  files just provide different defaults for the extension when run as a system
+  service. You can override anything from :file:`/etc/mopidy/extensions.d/` in
+  the :file:`/etc/mopidy/mopidy.conf` configuration file.
+
+- The init script runs Mopidy as the ``mopidy`` user. The ``mopidy`` user will
+  need read access to any local music you want Mopidy to play.
+
+- To run Mopidy subcommands with the same arguments, and thus the same
+  configuration files, as the init script uses, you can use ``sudo service
+  mopidy run <subcommand>``. In other words, where you'll usually run::
+
+      mopidy config
+
+  You should instead run the following to inspect the system service's
+  configuration::
+
+      sudo service mopidy run config
+
+  The same applies to scanning your local music collection. Where you'll
+  normally run::
+
+      mopidy local scan
+
+  You should instead run::
+
+      sudo service mopidy run local scan
+
+- Mopidy is started, stopped, and restarted just like any other system
+  service::
+
+      sudo service mopidy start
+      sudo service mopidy stop
+      sudo service mopidy restart
+
+- You can check if Mopidy is currently running as a system service by running::
+
+      sudo service mopidy status
+
+- Mopidy installed from a Debian package can use both Mopidy extensions
+  installed both from Debian packages and extensions installed with pip.
+  
+  The other way around does not work: Mopidy installed with pip can use
+  extensions installed with pip, but not extensions installed from a Debian
+  package. This is because the Debian packages installs extensions into
+  :file:`/usr/share/mopidy` which is normally not on your ``PYTHONPATH``.
+  Thus, your pip-installed Mopidy will not find the Debian package-installed
+  extensions.

docs/index.rst

@@ -37,6 +37,7 @@ Usage
     config
     running
     troubleshooting
+    debian
 
 
 .. _ext:

docs/installation/index.rst

@@ -55,6 +55,10 @@ in the same way as you get updates to the rest of your distribution.
    For a full list of available Mopidy extensions, including those not
    installable from apt.mopidy.com, see :ref:`ext`.
 
+#. Before continuing, make sure you've read the :ref:`debian` section to learn
+   about the differences between running Mopidy as a system service and
+   manually as your own system user.
+
 #. Finally, you need to set a couple of :doc:`config values </config>`, and then
    you're ready to :doc:`run Mopidy </running>`.
 

docs/running.rst

@@ -30,7 +30,8 @@ Init scripts
 
 - The ``mopidy`` package at `apt.mopidy.com <http://apt.mopidy.com/>`__ comes
   with an `sysvinit init script
-  <https://github.com/mopidy/mopidy/blob/debian/debian/mopidy.init>`_.
+  <https://github.com/mopidy/mopidy/blob/debian/debian/mopidy.init>`_. For
+  more details, see the :ref:`debian` section of the docs.
 
 - The ``mopidy`` package in `Arch Linux AUR
   <https://aur.archlinux.org/packages/mopidy>`__ comes with a systemd init