Add simpler top-level introductory definitions for EDVs and Identity Hubs

From the June25th call some introductory definitions/explanations for EDVs and Identity Hubs were shared and we should try to get something like this into the spec as front matter:

An EDV can be thought of as a "new storage primitive" based on client-side encryption and encrypted indexes ... that changes the trust characteristics around storage providers. An "identity hub"... is a thing that helps you manage N EDVs ... and also has a set of rules for responding to queries about the data residing on those EDVs.