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