updated preamble example in user manual

- resolves https://github.com/asciidoctor/asciidoctor-documentation/issues/17
- added code example to examples folder
- updated image to reflect code change
added header code examples to user manual

- nested doctitle section within header section
- created examples for doctitle and author info attribute
- added header metadata, keyword, and alt title section
- added sidebar about front matter
- moved front matter, doctype, backend, and docinfo file sections
- created images of rendered examples
- created examples for desc, keyword, and rev info attribute
- added header summary section
- added attributes to header summary table
- grouped and moved built-in data attributes
- resolves https://github.com/asciidoctor/asciidoctor-documentation/issues/16
use asciidoctor-docbot account for publishing
fix atom feed
Merge pull request #126 from graphitefriction/preamble-update

updated preamble example in user manual
Merge pull request #127 from graphitefriction/header-examples

added header code examples to user manual
Adds documentation for Asciidoctor-Java-Integration 0.1.4
AsciiDoc syntax updates and improvements

- switch to XML-friendly callouts
- use page-layout attribute for Awestruct layout
- use implicit table header and disable table caption globally
- add source language where missing
- use consistent AsciiDoc extension (.adoc)