This system allow containers, widgets and layouts to be listed in
the documentation with an example screenshot.
It uses raw HTML due to ldoc limitations, but as it is autogenerated,
it wont be user visible.
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>