This preprocessor adds support for several Markdown extensions that
should assist with writing.

The next commit will update to keep in sync with the reference.

This link section was already sorted but for a recent change (that
understandably probably preferred to keep the diff contained).  Let's
restore the sort order of the links here.

The CONTRIBUTING guide seems wrapped at 80 character lines.  Let's
rewrap a paragraph with a line that had overflowed.

Let's follow our own rules in the authoring guide by, e.g., using
`---` rather than the Unicode em-dash character, not wrapping lines,
specifying the language for code block sections, etc.

To improve formatting, we broke out some descriptions that were in
code block sections into the main text.  We replaced references to
"the spec" with "the reference" for consistency, as that's what most
of this document uses.  And we made some other small editorial
improvements.

These two items of style were the only bits present in the style guide
and missing from the authoring guide.  Let's copy them over, since
they are true of the Reference, and this will let us delete the style
guide entirely.

The authoring guide now fully incorporates and supersedes the style
guide, so let's simply link to that from the style guide.