Outline the major sections of the specification document

Goal: Create a high-level outline of the specification document. The outline should identify major topics (think level 1 and 2 section headings) and list them in their initial order. Deeper sections and rough notes about content can be included if they're easy to identify. This standalone document will be used for planning throughout the development of the specification.

To kick off the discussion, let's propose topics in the issue.