documentation recommendations

fixes AsciiDoc preview outside Gradle build (such as in Asciidoctor.js
Live Preview) by making the following changes:

- always set doctype in book master file (semantic integrity)
- always use fallback value for attributes used for resolution
- use blank lines around include directives to avoid unseen parse errors
- use blank lines around leveloffset entries to avoid unseen parse errors
Merge pull request #474 from mojavelinux/doc-recommendations

documentation recommendations