added preamble example

- fixed preamble section ID so toc link works
- added preamble.png image
- resolves https://github.com/asciidoctor/asciidoctor-documentation/issues/17
Merge pull request #117 from graphitefriction/preamble-example

added preamble example
Merge pull request #118 from graphitefriction/table-attr-additions

new content and examples for column specifier section
expanded table section of user manual

- added content and examples to cell specifier section
- expanded and formatted table summary section
- applies to issue https://github.com/asciidoctor/asciidoctor-documentation/issues/23