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.
"multiple versions" makes me think of object versioning that is available to end-users. I know it's not ... but it sounds that way.