WIP: Riak CS 1.5.0 docs

Addressing issue #1178. I'll reproduce the list of items in that issue here:

- [x] Multi-bag (multi-PB) support (tech preview)
- [x]  riak-cs-debug
- [x]  riak-cs-admin
- [x]  riak-cs-stanchion
- [ ] Syslog support through lager_syslog
- [ ] S3 API changes (COPY (tech preview), multi-DELETE, Cache Control headers)
- [x] Garbage collection improvements
- [ ] Configuration option(s) for features that can only be used with Riak 1.5
- [x] Update version compatibility page => #1063
- [ ] Fix CS install docs (especially for Mac OS X)
- [ ] New default value for ‘gc_paginated_indexes’

Time permitting
- [ ] Address ‘connection_pools’ attributes (https://github.com/basho/riak_cs/issues/752)
- [ ] Add kernel tuning advice (https://github.com/basho/riak_cs/issues/660)

https://github.com/basho/riak_cs/blob/release/1.5/RELEASE-NOTES.md#riak-cs-150-release-notes