Add download page for stable releases

The Apertis website currently points users to the images site for
downloading images. This site contains a lot of different images and
presents a daunting number of options for those not versed in Apertis
images (like new users).

Create a page on the Apertis website that provides a much easier way to get
the currently recommended (stable, ostree) images. Add links to images
using a shortcode to make the download page easier to update.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>