Improve user-manual html and pdf formatting

Use asciidoc style 'article' instead of 'book' and change asciidoc title level.
This removes blank first page and superfluous "Part I" page (there is no "Part II")
in pdf output. Also pdf size is decreased by this from 77 to 67 pages.
In html output this removes unnecessary sub-tocs and chapter numbering.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>