Instead of copying images to ${BUILD_DIR}/docs/images, which is the directory
with the input to ldoc, this now copies the images to ${BUILD_DIR}/doc/images,
which is where ldoc generates its output. That way, the images are together with
the html files and are automatically picked up by a lot of stuff.
Fixes: https://github.com/awesomeWM/awesome/issues/681
Signed-off-by: Uli Schlachter <psychon@znc.in>