* [Docs] Update and improve docs for Visualize and Discover
* Create a new section for default editor docs
* Fix significant terms link
* Writer changes
* Remove pages that aren't helpful to users
* More writer changes
This overhaul of the docs structure puts Kibana's documentation more
inline with the structure that is used in Elasticsearch. This will help
us better organize the docs going forward as more docs are added.
This also includes a few necessary content changes for 5.0.
