website: Fix anchors that were directing to "new.gem5.org"

The new.gem5.org website no longer exists. These incorrect anchors have
been changed to www.gem5.org.

Change-Id: I0aece2976ba15ea6d10feb6d1faa2b7f9be8225b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/42664
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>