Removed examples of Cray config files and added them under Spack on
Cray. Any users of Cray can just look at the Spack on Cray section to
get an idea of what's used. If they want more information they can click
links to the sections.
Updating how to use compilers.yaml on Cray as well as setting up
external packages. Also includes what needs to be set up for usage on
Cray and explains what needs to be done to get Spack properly working.
Also explain the architecture spec and what it does.
