downloads.html: Add a link to GPG key used signing releases
downloads.html: Add a link to GPG key used signing releases

While the key is available on public GPG key servers, having it locally
at https://libvirt.org/sources/gpg_key.asc is even better.

Signed-off-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt