Split docs into siteadmin and pluginwriter guides

* create initial bits for plugin writer's guide
* move siteadmin stuff to site administrator's guide
* rework index.rst to support multiple guides
* tweak some text
* move files into subdirectories

I verified that this still works with html and texinfo build targets.
There's still a lot of work to do, but this is a good start.

docs/source/siteadmin/plugins.rst

@@ -0,0 +1,137 @@
+=========
+ Plugins
+=========
+
+GNU MediaGoblin supports plugins that allow you to augment MediaGoblin's
+behavior.
+
+This chapter covers discovering, installing, configuring and removing
+plugins.
+
+
+Discovering plugins
+===================
+
+MediaGoblin comes with core plugins. Core plugins are located in the
+``mediagoblin.plugins`` module of the MediaGoblin code. Because they
+come with MediaGoblin, you don't have to install them, but you do have
+to add them to your config file if you're interested in using them.
+
+You can also write your own plugins and additionally find plugins
+elsewhere on the Internet. Once you find a plugin you like, you need
+to first install it, then add it to your configuration.
+
+.. todo: how do you find plugins on the internet?
+
+
+Installing plugins
+==================
+
+Core plugins
+------------
+
+MediaGoblin core plugins don't need to be installed because they come
+with MediaGoblin. Further, when you upgrade MediaGoblin, you will also
+get updates to the core plugins.
+
+
+Other plugins
+-------------
+
+If the plugin is available on the `Python Package Index
+<http://pypi.python.org/pypi>`_, then you can install the plugin with pip::
+
+    pip install <plugin-name>
+
+For example, if we wanted to install the plugin named
+"mediagoblin-restrictfive", we would do::
+
+    pip install mediagoblin-restrictfive
+
+.. Note::
+
+   If you're using a virtual environment, make sure to activate the
+   virtual environment before installing with pip. Otherwise the
+   plugin may get installed in a different environment than the one
+   MediaGoblin is installed in.
+
+Once you've installed the plugin software, you need to tell
+MediaGoblin that this is a plugin you want MediaGoblin to use. To do
+that, you edit the ``mediagoblin.ini`` file and add the plugin as a
+subsection of the plugin section.
+
+For example, say the "mediagoblin-restrictfive" plugin had the Python
+package path ``restrictfive``, then you would add ``restrictfive`` to
+the ``plugins`` section as a subsection::
+
+    [plugins]
+
+    [[restrictfive]]
+
+
+Configuring plugins
+===================
+
+Configuration for a plugin goes in the subsection for that plugin. Core
+plugins are documented in the administration guide. Other plugins
+should come with documentation that tells you how to configure them.
+
+Example 1: Core MediaGoblin plugin
+
+If you wanted to use the core MediaGoblin flatpages plugin, the module
+for that is ``mediagoblin.plugins.flatpages`` and you would add that
+to your ``.ini`` file like this::
+
+    [plugins]
+
+    [[mediagoblin.plugins.flatpages]]
+    # configuration for flatpages plugin here!
+    directory = /srv/mediagoblin/flatpages
+
+Example 2: Plugin that is not a core MediaGoblin plugin
+
+If you installed a hypothetical restrictfive plugin which is in the
+module ``restrictfive``, your ``.ini`` file might look like this (with
+comments making the bits clearer)::
+
+    [plugins]
+
+    [[restrictfive]]
+    # configuration for restrictfive here!
+
+Check the plugin's documentation for what configuration options are
+available.
+
+
+Removing plugins
+================
+
+To remove a plugin, use ``pip uninstall``. For example::
+
+    pip uninstall mediagoblin-restrictfive
+
+.. Note::
+
+   If you're using a virtual environment, make sure to activate the
+   virtual environment before uninstalling with pip. Otherwise the
+   plugin may get installed in a different environment.
+
+
+Upgrading plugins
+=================
+
+Core plugins
+------------
+
+Core plugins get upgraded automatically when you upgrade MediaGoblin
+because they come with MediaGoblin.
+
+
+Other plugins
+-------------
+
+For plugins that you install with pip, you can upgrade them with pip::
+
+    pip install -U <plugin-name>
+
+The ``-U`` tells pip to upgrade the package.