We might choose not to duplicate the section lists within each part as shown above, since the index page for the part already includes that information; but I think it’s more friendly this way, despite being more of a maintenance headache.
We might also choose to completely break with the layout of the PRMs though and go with a more free-form and interlinked approach, as in the Rails API or the Apple Developer Connection documentation. The trouble is, this would most likely require a huge amount of manual work with little opportunity for automation.