skipping heading levels (eg from # to ###, or starting at ###) is legal
in pandoc, but not in nixos-render-docs. pandoc acts as though section
levels *were* consecutive, nixos-render-docs prefers to tell people not
to do that kind of thing because it can make documents more fragile.
pandoc drops .title classes when rendering to docbook, so these are
effectively just paragraphs anyway. without support for including them
in a table of contents the complexity of parsing them in
nixos-render-docs won't be warranted.
