Add a rule to optionally build docs with the stylesheet from the website

For those of us who prefer the formatting of the docs using the
website stylesheets. Use "make STYLE=website draft" (for example) to use.

The stylesheet itself is referenced directly to the website, so there
is currently no copy of it stored in the source repository. Thus, docs
built with it will only look correct if the browser can access the website
when viewing them.