website: Update homepage

This adds a link to the most recent blog post and the gem5-20 paper
to the homepage.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: I1918169d096282f0f3cbafba4bac8294f51b2bc1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/35441
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>