added docinfo header and footer section

- how to create docinfo file
- how to name docinfo file
- set docinfo attributes
- substitute attributes in docinfo file
- related to #88
- related to https://github.com/asciidoctor/asciidoctor/issues/116
- related to https://github.com/asciidoctor/asciidoctor/issues/403
- related to https://github.com/asciidoctor/asciidoctor/issues/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.