[DOCS] Expand scripting docs to include Painless elements

[lockewritesdocs]

#69748 refocused the scripting content on Painless and provided an introduction to writing scripts. This content is focused on both novice and expert programmers who are novice Painless users. With the initial groundwork complete, we need to expand the scripting docs to introduce users to the major elements of Painless in a way that caters to this audience.

This proposal is to explain the following elements after the section on How to write scripts. The Scripting team discussed using the Painless test API as the example context to use throughout these sections:

• Types
• Variables
• Statements
• Functions

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[jdconrad]

Our docs have been completely rewritten.