This is a first attempt at giving Fedora Documentation Writers a one-
stop location to get started writing AsciiDoc. Instead of trying to
rewrite existing documentation about AsciiDoc, I linked out to helpful
resources maintained by the asciidoctor.org upstream.

However, this commit does write new documentation about handy snippets
to reuse in a Fedora-specific context. I also wrote a short guide on how
to use reusable attributes, since I realize this is something I have
done in many repositories I set up but never documented.

Part of #8 to improve the new contributor experience to Fedora
Documentation with AsciiDoc and Antora.