added docinfo header and footer section

[graphitefriction]

• how to create docinfo file
• how to name docinfo file
• set docinfo attributes
• substitute attributes in docinfo file
• related to Update new features into AsciiDoc Syntax Reference #88
• related to Include docinfo files into header asciidoctor#116
• related to Substitute attributes in docinfo files asciidoctor#403
• related to Add docinfo equivalent for footer asciidoctor#486

Next actions:

1. The bulk of the docinfo content should probably be moved to the producing or rendering part of the user manual, with just a brief description and a link included in its current place.
2. Create transitions between the sections.
3. Create content about what a footer can contain in general - not just in the sense of a docinfo file.
4. Remove the table captions from the entire user manual.

[mojavelinux]

Most excellent. It's so hard to explain this on the fly...and it never comes out right, so being able to point users here will really speed things up!