Better indentation for Network Guide chapter "under the hood"

Bug #1211658 reports that the reader was confused that we have
several times "Scenario 1". This is a result of a flat structure, I
grouped the chapter now with better sections and subsections.

Patchset 3: Add dbhtml stop-chunking - thanks Anne for the suggestion.

Patchset 4: Takes in editorial suggestions from Diane (thanks!)

Change-Id: I37d5500b818ad0eed6a4f6fe29317e8dd4134021
Closes-Bug: #1211658