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.
initial wording fixes
more wording changes
Correct version metadata for garbage collection document
Updates to address review feedback
Correct documentation on GC pause and add documentation for cancel command
initial changes
fix gc_paginated_index version issue
incorporate 1.5.0+ sections
add gc_max_workers config
Merge pull request #1198 from basho/feature/klm/cs-gc-documentation

Expand Riak CS garbage collection documentation