Don't promote daily images and set expectations for booting in VirtualBox

We link to the hardware reference page from the home page, as a result this
page is likely to be one which is visited by people generally unfamilar with
Apertis.

We should not promote the use of daily images to those likely to be looking to
evaluate Apertis - they should be using the stable images.

Booting non-SDK images in VirtualBox is not a supported option, so whilst it
works for many images, this is not guaranteed (and the minimal OSTree images
are known not to boot in VirtualBox). Set expectations with regards to running
in this configuration.

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