also updates nixdoc to 2.3.0. the nixdoc update is not a separate commit
because that would leave the manual build broken for one commit,
potentially breaking bisects and rebases.
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.
