The ordering examples use `Customer#orders_count` for ordering which
results in beginner-unfriendly examples like:

  Customer.order(:orders_count)

Having two unrelated types of `order` can be confusing.
As Customer has many Orders it's probably better to replace the Customer
with Book to avoid any confusion.

Not having `orders_count` also makes it easier to grep for 'order' in
the guide. This also is a reason the `Order.none` example is replaced
with `Book.none`, besides it being more consistent with the example
below it.

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>