code: function lookupStructureFile(book, type) {
      var config = book.getConfig();
  
      var fileToSearch = config.getValue(['structure', type]);
  
      return findParsableFile(book, fileToSearch);
  }
docs: Lookup a structure file (ex: SUMMARY.md, GLOSSARY.md) in a book. Uses
  book's config to find it.
  
  @param {Book} book
  @param {String} type: one of ["glossary", "readme", "summary", "langs"]
  @return {Promise<File | Undefined>} The path of the file found, relative
  to the book content root.