make beaker-job.rng a Sphinx download

This keeps it versioned with the Beaker branch where it came from, and
also lets us avoid the copying step in the web site build.

One day in the distant glorious future, we will have a Sphinx plugin to
convert the schema into some human-readable form also...

Bug: 980340
Change-Id: Ie0f6d7784c4301dc220df281767c015aa769969b