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.
This version is just geared at the 1.4 series. I have a separate branch that documents the 1.5 changes, but I have not opened a PR for it yet.