Issue: Leverage Caching in Books

Whether it appears in a chapter or on the book's index, the book TOC is a potentially long list with multiple levels that takes at least 2 queries in VaultWiki 3, and more like 5 in VaultWiki 4, to fully construct.

Further, pages that are chapters take between 0 and 4 queries to form the previous/next chapter links in VaultWiki 3, and potentially more than that in VaultWiki 4.

For both of these, the results are fairly static as the chapter order is not dependent on things like last-update times. These should be cached using VaultWiki 4's block cache (though a slight stretch to call the nav links a block) so that these expensive links are only formed once (never expires) and the cache is only cleared if the chapters in the book are added, removed, reordered, or renamed.