Restructing: Fix the part/chapter structure of the books

[dedemorton]

Right now, the Beats books are not ideally structured because the structure has grown organically over time. To save time and minimize the amount of rework required, we've compromised and modified the existing structure to meet our needs. This has resulted in a rather strange book structure that begins as single chapters and morphs into sections halfway through the book. The parts are necessary (given the current heading levels and chunk setting) to achieve the levels of nesting required, but they make maintaining the book over time (and reusing content) more challenging.

Ideally, we should strive for a simple structure where there is a one to one mapping between each source file and the topic that gets built from it. We should use a simple topic hierarchy and leverage the chunk setting to create the topic hierarchy we require.

This is a significant effort and will result in massive diffs. This effort is best left for a time when we are updating to a new format (such as ascidoctor if we go that direction) or making a significant change to the information architecture of our books (for example, if we go to one Beats book rather than several).

[dedemorton]

Closing this issue because we will make the minimal changes required to get the book building in asciidoctor.