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.
Welcome to the least understood field in all of docs :)

`version` represents to the oldest version of the *document*, starting with the most recently deployed doc version. It doesn't have anything to do with the age of the *content* written about. So while the text may apply for 1.2.0+, the doc didn't exist until 1.4.5+, it should be 1.4.5+.