website: Update GCN3 docker image + square documentation

* Add documentation stating there's a pre-built docker image and how
to use it

* Update documentation regarding how to run square, as the structure
of gem5-resources changed since the documentation was written

Change-Id: I6b0cbdf6c191c8584edf9a34deaf916f969ec6ff
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/43565
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed