Improve Docker documentation

* Add “Containers” in nav for more details
* Shrink headers from `<h2>` to `<h3>` where appropriate for better
  segmentation (all headers had the same weight previously)
* Remove some Mesos implementation details that are not relevant to a
  user like protobuf changes
Merge pull request #544 from mesosphere/docker-doc-upgrades

Improve Docker documentation