FIgure out what to do with `chapter_titles`

[ehuss]

RenderContext::chapter_titles and Preprocessor::chapter_titles provide a way for the custom chapter titles (provided by {{# title}}) to be ferried to the HTML renderer. This creates some awkwardness:

• This is an internal field that unfortunately gets publicly exposed.
• Slightly awkward with the serde skip (since it is a field the user can't really use).
• It's not documented terribly well.

I'm not sure what, if anything to do here.