We made a mistake in opening up Kibana scripted fields to any and all
langs enabled in Elasticsearch. Kibana needs to do some magic to get
certain scripted field features to work
(https://github.com/elastic/kibana/pull/9171) and we can't commit to
supporting anything beyond expression and painless scripts. On top of
that, the other languages are being removed in ES 6.0 anyway. This commit
makes that stance explicit by disallowing anything other than those two
choices.
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.
