Authoring Markdown for md2html

No md2html on PATH? Get it from
github.com/AdamF-G/md2html:
go install github.com/AdamF-G/md2html/cmd/md2html@latest (needs Go 1.26
or later).

Run md2html --guide and read its output before writing the page. It
prints the authoring guide's quick reference and silent traps for the
binary that will convert the page, so it cannot describe a different
version, and ends with an index of the guide's other sections. Before using
a construct beyond a plain line in the quick reference, such as a
container, a fig diagram or a cross-reference, print its section:
md2html --guide callouts-and-containers, md2html --guide diagrams. If
the conversion prints warnings, md2html --guide warnings says what each
one means and how to fix it. In the md2html repo the whole guide is
docs/authoring.md.

Three of those traps come up most. Each gives valid output and a zero exit,
with no warning:

• A fence inside a fence needs a longer outer fence. Showing
  three-backtick source takes four backticks around it; showing a :::
  container inside a container takes :::: on the outer one.
• Link to the .md source, never the .html. An .html link is never
  matched to the page md2html emits, and when the file exists nothing
  warns.
• Only the first class names a container's kind. ::: {.compact .warning} is a plain, unstyled div; write ::: warning {.compact}.