Update documentation about generated test cases
Update documentation about generated test cases

The various file names and file paths have changed.

This breaks the documentation into three logical sections targeting
the tasks that the reader will likely want to choose from:

* regenerating the exercise
* changing a test suite
* implementing a new generator

Insti/xruby,Insti/exercism-ruby,Insti/exercism-ruby,NeimadTL/ruby,exercism/xruby,Insti/xruby,NeimadTL/ruby,NeimadTL/ruby,exercism/xruby,Insti/exercism-ruby