release: add 14.0-BETA4 download links to website

Uncomment the 'helptest' section.

Notably, the armv6 release images are gone with the 14.0 release.

This section was kept up-to-date during the 13.2 release cycle by
dbaio@. I would be happy to do it this time around.

Reviewed by: lwhsu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42036