website: update GCN3 documentation

This commit updates the GCN3 GPU documentation to make it clear
that the GPU model is currently only supported on the develop
branch, not stable.

Change-Id: Iab4d07f97edeff37384d2a0adbfb8a402df90a56
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/42803
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed