stages docbook and xhtml 1.5 changes

- adds xhtml file
- updates render docbook section
- adds xhtml section to user manual
- related to #158
- documents asciidoctor/asciidoctor#494
- documents asciidoctor/asciidocto#554
Merge pull request #238 from graphitefriction/1-5-docs

stages docbook and xhtml 1.5 changes
draft of first version of asciidoctor diagram docs

- TODO generated out file structure
- TODO sizing and positioning of image
- TODO link doc with other docs
Merge pull request #239 from graphitefriction/diagram

draft of first version of asciidoctor diagram docs