Task to generate Painless API's per context (#41233)

This adds a gradle task called generateContextDoc in the Painless module. The 
task will start a cluster, issue commands against the context rest api for 
Painless, and generate documentation for each API per context. Each context 
has a first page of classes sorted by package first and class name second, 
along with a page per package with each classes' constructors, methods, and 
fields. A link is generated for each constructor, method, and field to a JavaDoc 
page when possible.

Author: jdconrad

docs/Versions.asciidoc

@@ -30,8 +30,7 @@ release-state can be: released | prerelease | unreleased
 ///////
 Javadoc roots used to generate links from Painless's API reference
 ///////
-:java8-javadoc: https://docs.oracle.com/javase/8/docs/api
-:java9-javadoc: https://docs.oracle.com/javase/9/docs/api
+:java11-javadoc: https://docs.oracle.com/en/java/javase/11/docs/api
 :joda-time-javadoc: http://www.joda.org/joda-time/apidocs
 :lucene-core-javadoc: http://lucene.apache.org/core/{lucene_version_path}/core
 

docs/painless/painless-api-reference.asciidoc

@@ -1,13 +1,11 @@
 [[painless-api-reference]]
 == Painless API Reference
 
-Painless has a strict whitelist for methods and classes to ensure all
-painless scripts are secure. Most of these methods are exposed directly
-from the Java Runtime Environment (JRE) while others are part of
-Elasticsearch or Painless itself. Below is a list of all available
-classes grouped with their respected methods. Clicking on the method
-name takes you to the documentation for that specific method.  Methods
-defined in the JRE also have a `(java 9)` link which can be used to see
-the method's documentation in Java 9.
+Painless has a strict whitelist for methods and classes per context to
+ensure all painless scripts are secure. Most of these methods are
+exposed directly from the Java Runtime Environment (JRE) while others
+are part of Elasticsearch or Painless itself. Below is a list of the available
+APIs per context. The shared API is available to all contexts, while the
+specialized API available differs between contexts.
 
 include::painless-api-reference/index.asciidoc[]