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: Simon Glass <sjg@chromium.org>