Revised RDoc for Ruby CSV

I've completed a major revision of the RDoc for Ruby class CSV, along with its nested classes CSV::Table and CSV::Row. This is not yet merged into the master branch at ruby/ruby, but may be seen on ruby.github.io at the links above.

I've also been experimenting with CSV recipes, which focus on specific user tasks rather than on API capabilities.

Oh, and here's the existing RDoc.