This adds the `ToUrlPath` helper trait to convert a Path to a path
suitable for use in HTML (replacing `normalize_path`).
This also fixes a minor bug where on Windows the next/prev links were
using a double forward slash. I don't think this is possible, since
chapter links are derived from the summary, but I'm noting just in case.
It's also not too much of an issue since double slashes are normally
just treated as a single.
