document design principles for hub and spoke model for coco-pattern

[butler54]

In discussions with @sabre1041 @yuvalk and others a high level pattern has emerged

1. ACM hosting (aka HUB) cluster hosts Trustee. Design point for this cluster is to be hardware invariant
2. Coco supporting (via peer pods or normal kata) occurs on a spoke
3. ACM is used to propagate necessary information to spoke specifically:
   1. Trustee address
   2. Trustee CA chain
4. Hardware platforms are dealt with as different 'cluster groups' allowing users to make assumptions within the cluster group on variables.