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.
In 1.5.0, this value is used to divide keys to chunks to pass workers even `gc_paginated_indexes=false`.
https://github.com/basho/riak_cs/blob/release/1.5/src/riak_cs_gc_key_list.erl#L108