CAO: Add examples of Pod placement / Node Selection

The CAO docs cold be improved with the addition of some examples/tutorials around node placement/hardware selection, particularly in the area of MDS (e.g. using different Node Pools to provide Pods for different Couchbase Server services.

As a minimum, augmenting the example in Configure Multi-Dimensional Scaling, and showing how to make use of Node Selectors in couchbaseclusters.spec.servers.