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