[DOCS] Add docs-dir to Painless (#25482)

Author: lcawl

docs/painless/index.asciidoc

@@ -1,7 +1,7 @@
 [[painless]]
 = Painless Scripting Language
 
-:ref: https://www.elastic.co/guide/en/elasticsearch/reference
+:docs-dir:  {docdir}/../../../docs
 
 include::../Versions.asciidoc[]
 
@@ -20,7 +20,7 @@ include::painless-api-reference.asciidoc[]
 ////
 Proposed Outline (WIP)
 Getting Started with Painless
-  Accessing Doc Values 
+  Accessing Doc Values
   Updating Fields
   Working with Dates
   Using Regular Expressions