* Update the top-level 'getting started' guide.
* Remove custom types from the painless getting started documentation.
* Fix an incorrect references to '_doc' in the cardinality query docs.
* Update the _update docs to use the typeless API format.
Currently failures to compile a script usually lead to a ScriptException, which
inherits the 500 INTERNAL_SERVER_ERROR from ElasticsearchException if it does
not contain another root cause. Instead, this should be a 400 Bad Request error.
This PR changes this more generally for script compilation errors by changing
ScriptException to return 400 (bad request) as status code.
Closes#12315