The main way CONTRIBUTING will be viewed is on Github and the output
is just not good enough to justify using asciidoctor vs md/txt as
the recommended on the Github help page for contributing guidelines.

For example it was attractive to use a TOC and that works on Github
but doesn't look good. In the end it's a very short TOC and
maintaining manually is easy enough and it's easier to control the
look.

Essentially optimize for viewing on Github since the Asciidoctor
TOC works but doesn't look good with no styling.

The guidelines were originally created several years ago. This commit
does not fundamentally change anything but rather make the guidance
more compact.

The commit consolidates into into CONTRIBUTING.adoc information from
CONTRIBUTING-DOCUMENTATION.adoc and also the wiki page "The Lifecycle
of an Issue".