Redo the section layout

* Fix the Packt style used
* Pull out chapter number as a section and instead rely on attribute :chapter_number, defaulting to **1** if not specified
* Move everything up, so that main sections are Level 1, i.e. `==`, etc.
* Add a preamble

Issues:

* Can't seem to grab the title inside the preamble. docbook's block_preamble.xml.slim seems to use it. Why can't I?
* If the Chapter Title styling works, do I need to then rip it's relic out of section.fodt.slim?