doc: Move coccinelle into its own section

This tool has nothing to do with testing. It is for refactoring code
automatically using a 'semantic patch' tool.

Create a new section for 'refactoring' and move it into there. It is
likely that other topics may fall under the same heading, such as
using moveconfig and search/replace tools.
Signed-off-by: NSimon Glass <sjg@chromium.org>

doc/develop/index.rst

@@ -42,7 +42,14 @@ Testing
    :maxdepth: 1
 
    testing
-   coccinelle
    py_testing
    tests_writing
    tests_sandbox
+
+Refactoring
+-----------
+
+.. toctree::
+   :maxdepth: 1
+
+   coccinelle