Add a new doc build target called 'prettify'.
This calls the style/xsl/list_for_prettify.xsl file added in r1828598.

This is a convenient way to prepare things to be (manually) updated in
prettify.js in order to upgrade syntax highlight in the doc.

git-svn-id: https://svn.apache.org/repos/asf/httpd/docs-build/trunk@1828599 13f79535-47bb-0310-9956-ffa450edef68