Add the nuance of "the edge" to the overview

[dovholuknf]

The edge part of a ziti overlay network is largely glossed over - mostly for a good reason. initial learners have a lot to digest as is. However - once someone starts looking into 'terminators' the nuance begins to be more important.

update the overview with some broad words about the edge and update more details about what 'the edge' means and how it's different (but incredibly similar to) 'the fabric'. Possibly include definition of terminators in the work. here's some words that might help frame terminators.

terminators are metadata stored in the controller which are used during path selection. these terminators represent all the possible routers where traffic can leave the openziti fabric. leaving the fabric does not necessarily mean leaving the overall, overlay network. If the final destination of the traffic is an edge component, the traffic continues on the ziti edge overlay network to the actual destination of the traffic.