This commit cleans up the README and splits portions of it out into
a user guide (GUIDE.md) and a FAQ (FAQ.md). The README now provides a
small list of documentation "quick" links to various parts of the docs.
This commit also does a few other minor touchups.
This commit uses the recent refactoring for defining flags to
automatically generate a man page. This finally allows us to define the
documentation for each flag in a single place.
The man page is generated on every build, if and only if `asciidoc` is
installed. When generated, it is placed in Cargo's `OUT_DIR` directory,
which is the same place that shell completions live.