Rename Book.sections to Book.items

This renames the "sections" list to "items". In practice, this list has
contained more than just "sections" since parts were added. Also, the
rest of the code consistently uses the term "items", since the values it
contains are called `BookItem`s. Finally, the naming has always been a
little confusing to me.

This is a very disruptive change, and I'm not doing it lightly. However,
since there are a number of other API changes going into 0.5, I think
now is an ok time to change this.

guide/src/for_developers/preprocessors.md

@@ -91,7 +91,7 @@ if __name__ == '__main__':
     # load both the context and the book representations from stdin
     context, book = json.load(sys.stdin)
     # and now, we can just modify the content of the first chapter
-    book['sections'][0]['Chapter']['content'] = '# Hello'
+    book['items'][0]['Chapter']['content'] = '# Hello'
     # we are done with the book's modification, we can just print it to stdout, 
     print(json.dumps(book))
 ```