Document how the different flavors of customer-managed agents work

[komalali]

A customer had a lot of questions about the differences in how the k8s vs DinD customer-managed agents work. We don't have any documentation that specifically explains the internals of how these two configurations work, and the differences between them. Specifically, questions about scaling, the 1:1 relationship between agent and running job, etc. came up - which would be useful to cover.