Consistent layout and headings for devref

Consistent layout and headings for devref

The lack of convention for heading levels among the independently
written devref documents was starting to make the Table of Contents
look rather messy when rendered in HTML.

This patch does not cover the "Neutron Internals" section since its
layout is reasonably OK for now.

Change-Id: I827c105599f05773bda7e4fc0a941ce04ebd51fa