adds source code block content

- documents uri attr asciidoctor/asciidoctor#800
- documents backslash asciidoctor/asciidoctor#1022
- documents source language asciidoctor/asciidoctor#888
- removes duplicates source code examples files
- updates include files for source code examples
- changes include file names for desc and keywords
Merge pull request #243 from graphitefriction/1-5-docs

adds source code block content