Add documentation and CI schema for variables:expand/raw

What does this MR do and why?

This MR adds documentation and CI schema for variables expand/raw.

You can also find all example details in the MRs below 👇

Step	Status
Add expand syntax to CI config	!98420 (merged)
Implement expand/raw logic for basic jobs	!102536 (merged)
Add QA test for raw variables in CI config YAML	!103181 (merged)
Use expand/raw logic for variables passed by bridge/trigger	!103190 (merged)
Add documentation and CI schema for expand/raw	<- HERE

Related to #353991 (closed)

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