Index part titles when doctype is book

Currently, part titles are not being indexed. If a page declares the doctype as "book", then it can have multiple h1 tags. Currently, the indexer ignores these, starting at h2 instead.

The indexer is also too lenient with how it identifies the page title. It should look for h1.page (and only the first occurrence) rather than just the first h1. (We'll need to document this assumption in the docs here and in the default UI).

I think both of these issues can be addressed in a single change.