build system: fix parallel build of docs

Before this commit, each html file we built would copy its images from the
docs source directory to the docs html (target) directory.

Some of our html documents share images with other html documents, for
example many of the translations.

With 'make -j', and since all the html targets are built in parallel,
sometimes two make processes would try to copy the same image file at
the same time, resulting in failure.

This commit changes how images are handled.  All the html documents
are still handled in parallel, but the image copying is deferred until
after the html docs are all done, then all the images are copied by a
single thread.