docs: admin-guide: hw_random: update rng-tools website

rng-tools upstream moved to github. New upstream does not appear to
consider itself official website for hw_random. Drop that part.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/ef52ace5008fa934084442149f64f5f9ddbba465.1704720105.git.baruch@tkos.co.il