website: add pointers to the PGP signatures for releases

Peter has just pointed me to the PGP signatures of the release tarballs,
which exist since he's been doing releases.

This was not obvious to me that the releases were signed, as there was
no info about it on the website.

So, add such links on the downloads page.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>