Chapter toc 62963228

It was bothering me that frontmatter elements like the preface weren't linked in the Table of Contents, but that's how LaTeX does it—except that it doesn't. In fact, LaTeX does it the right way. This PR, together with [this one](https://github.com/softcover/softcover/pull/81), syncs up the other formats with PDF. The result is nice prominent links to any foreword, preface, etc., on `/book`, making it much less likely that readers will accidentally miss them.