doc: Add "extensions" to usage guide

Signed-off-by: Stephen Finucane <stephen@that.guru>

doc/usage/extensions/imgconverter.rst

@@ -0,0 +1,40 @@
+.. _sphinx.ext.imgconverter:
+
+:mod:`sphinx.ext.imgconverter` -- A reference image converter using Imagemagick
+===============================================================================
+
+.. module:: sphinx.ext.imgconverter
+   :synopsis: Convert images to appropriate format for builders
+
+.. versionadded:: 1.6
+
+This extension converts images in your document to appropriate format for
+builders.  For example, it allows you to use SVG images with LaTeX builder.
+As a result, you don't mind what image format the builder supports.
+
+Internally, this extension uses Imagemagick_ to convert images.
+
+.. _Imagemagick: https://www.imagemagick.org/script/index.php
+
+.. note::
+
+   Imagemagick rasterizes a SVG image on conversion.  As a result, the image
+   becomes not scalable.  To avoid that, please use other image converters like
+   `sphinxcontrib-svg2pdfconverter`__ (which uses Inkscape or
+   ``rsvg-convert``).
+
+.. __: https://github.com/missinglinkelectronics/sphinxcontrib-svg2pdfconverter
+
+
+Configuration
+-------------
+
+.. confval:: image_converter
+
+   A path to :command:`convert` command.  By default, the imgconverter uses
+   the command from search paths.
+
+.. confval:: image_converter_args
+
+   Additional command-line arguments to give to :command:`convert`, as a list.
+   The default is an empty list ``[]``.