website: Add badges for build status

Adds three badges, one for nightly, weekly, and compiler tests to the
homepage.

See https://imgur.com/mCNZoUl for an example of what the new page looks
like.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: Id5a28cd31deb91f80001df741f681685db20c3b0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/51467
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>