doc: Add "markdown" section to usage guide

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

doc/config.rst

@@ -136,7 +136,8 @@ General configuration
 
    .. note::
 
-      Read more about how to use Markdown with Sphinx at :doc:`/markdown`.
+      Refer to :doc:`/usage/markdown` for more information on using Markdown
+      with Sphinx.
 
    .. versionadded:: 1.3
 

doc/contents.rst

@@ -10,6 +10,7 @@ Sphinx documentation contents
    usage/installation
    usage/quickstart
    usage/restructuredtext/index
+   usage/markdown
 
    intro
    man/index
@@ -20,7 +21,6 @@ Sphinx documentation contents
    setuptools
    templating
    latex
-   markdown
    extensions
    extdev/index
    websupport