website: Add a note on CPU/ISA independence Change-Id: Ie44f8f437895cddddecaf9b0c5baf74eb657ae1b Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/62072 Maintainer: Bobby Bruce <bbruce@ucdavis.edu> Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu> Tested-by: Bobby Bruce <bbruce@ucdavis.edu>