Expand Riak CS garbage collection documentation

* Add a document detailing the function of the Riak CS garbage
collection system that was modified from and existing document on the
riak_cs repo wiki.
* Add a link to the new garbage collection document to the global navigation.
* Document the configuration options available for the Riak CS garbage
  collection system.
I'm always confused between 1.4.x and 1.5 :sweat_smile: . 
This (epoch + leeway_seconds) is 1.4.5 behavior.
(1.5 uses just epoch).