created deckjs backend guide

- instructions for using custom templates
- split example and backend attribute
- block step example
add url alias to quick reference
Merge pull request #70 from graphitefriction/deckjs-guide

created deckjs backend guide
Added link to deckjs backend guide
Added links to install guides
Corrected cross-ref syntax
Use actual gem names
Clarify backend attribute value
Lowercase deck.js
Qualify the backend as being for Asciidoctor
Cleanup opening sentence
Reword guide reference
notes [ci skip]
add missing table cell styles