Downloads page: use PDF artifact (rebased onto dev_4_4)



This is the same as gh-685 but rebased onto dev_4_4.

----

With the documentation PDF being systematically built and copied over during the release process, using this artifact in the downloads page should simplify the linkchecking.

To be tested as part of the http://hudson.openmicroscopy.org.uk/job/BIOFORMATS-trunk-release job