Add arm/armv6 SD Card Image link.
Add amd64 and i386 VM images link.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
- remove deprecated gnats from pr-guidelines article

PR:		194787
Differential Revision:	https://reviews.freebsd.org/D1121
Reported by:	emaste
Approved by:	remko (mentor)