Arch Design: Move introductory sections up

Move the introductory sections one level up, removing the separate
section for them. This reads much nicer in HTML now.
There's also no need to name the first paragraphs of a chapter
"Introduction", this is implied.

Change-Id: Ife4275807561dae2ca57dc71b7602240efa94663
Merge "Arch Design: Move introductory sections up"