release/1.6.0 site config & documentation updates for Orion, Derecho, Narwhal/GNU, AWS ParallelCluster, AWS Single-Node AMI Red Hat 8, Casper

release/1.6.0 site config & documentation updates for Orion, Derecho, Narwhal/GNU, AWS ParallelCluster, AWS Single-Node AMI Red Hat 8, Casper #855