nixos-render-docs: add manual html converter

this converter is currently supposed to be able to reproduce the
docbook-generated html DOMs exactly, though not necessarily the
html *files*. it mirrors many docbook behaviours that seem rather odd,
such as top-level sections in chapters using the same heading depth as
understood by html as their parent chapters do. over time we can
hopefully remove all special casing needed to reproduce docbook
rendering, but for now at least it doesn't hurt *too* much.