feat: book menu - link to book of books if exists
feat: book menu - link to book of books if exists

Example: `courses/learn-x/` will have a new item added to the book menu to link back to `courses/` whereas a site with just a first-level book e.g. `docs/` won't see any change to the menu.

gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic