Rename dev subdirectory to plugin-development

This should make the contents clearer

docs/source/dev/formatters.rst

@@ -1,54 +0,0 @@
-.. _formatting-plugins:
-
-===========================================
- Developing a Formatting Plugin for Flake8
-===========================================
-
-|Flake8| allowed for custom formatting plugins in version
-3.0.0. Let's write a plugin together:
-
-.. code-block:: python
-
-    from flake8.formatting import base
-
-
-    class Example(base.BaseFormatter):
-        """Flake8's example formatter."""
-
-        pass
-
-We notice, as soon as we start, that we inherit from |Flake8|'s
-:class:`~flake8.formatting.base.BaseFormatter` class. If we follow the
-:ref:`instructions to register a plugin <register-a-plugin>` and try to use
-our example formatter, e.g., ``flake8 --format=example`` then
-|Flake8| will fail because we did not implement the ``format`` method.
-Let's do that next.
-
-.. code-block:: python
-
-    class Example(base.BaseFormatter):
-        """Flake8's example formatter."""
-
-        def format(self, error):
-            return 'Example formatter: {0!r}'.format(error)
-
-With that we're done. Obviously this isn't a very useful formatter, but it
-should highlight the simplicitly of creating a formatter with Flake8. If we
-wanted to instead create a formatter that aggregated the results and returned
-XML, JSON, or subunit we could also do that. |Flake8| interacts with the
-formatter in two ways:
-
-#. It creates the formatter and provides it the options parsed from the
-   configuration files and command-line
-
-#. It uses the instance of the formatter and calls ``handle`` with the error.
-
-By default :meth:`flake8.formatting.base.BaseFormatter.handle` simply calls
-the ``format`` method and then ``write``. Any extra handling you wish to do
-for formatting purposes should override the ``handle`` method.
-
-API Documentation
-=================
-
-.. autoclass:: flake8.formatting.base.BaseFormatter
-    :members: