website: Update gem5 reference for gem5-20

This change also adds a bibtex file to make it easier to cite gem5-20.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: Ic2204c4fb5a6a6c2fb65955fb8f4e6dbd5e1d098
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/51267
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
2 files changed