Update docs on how to add and test a new system

[pearce8]

@dyokelson you had these nice instructions for testing Fugaku; please make sure we have them in the docs.

Test system setup with:

cd benchpark
. setup-env.sh
benchpark system init --dest fugaku-system fugaku 

Visually validate files created in fugaku-system directory

Run an existing experiment:

benchpark experiment init --dest saxpy-test saxpy +openmp
benchpark setup ./saxpy-test ./fugaku-system workspace/

Then follow with the ramble commands that are output.