Document structure

[martinthomson]

Discussed with editors and we tentatively agreed about a plan for document structure.

Right now we have have a section on API that includes both usage and implementation internals. We agreed to split those into two sections so that we can present usage up front. The API internals section can include data structures and algorithms, plus maybe permissions policy integration and other detailed stuff that users likely don't care about much. (We'll need to review this again, likely, but this should give us a place to start.)

[csharrison]

I'd argue that permissions policy should go into the "usage" section as it is caller-facing.