Update "Download Haiku" graphic for beta2

* beta1 -> beta2
* rotate the "beta2" stamp as it was remarked before that the stamp
going 'uphill' is supposed to have better vibes than 'downhill' as
it's now...

Change-Id: I60a60a1ba36ac11817294c91deecc555582e039e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2697
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Improved "Get Haiku" download graphic

Added "Get Haiku" beta banner

It's my first shot at it (actually second shot after jessicah
shot down the first one :) ). Feel free to improve.